Technical SEO

5xx server errors are HTTP status codes from 500 to 599.

The status code 200 OK means that an HTTP request has succeeded.

304 Not Modified is an HTTP status code that tells a client its cached copy of a resource is still current, so the server does not send …

The HTTP status code 401 Unauthorized means a request lacks valid authentication credentials for the requested resource.

403 Forbidden is an HTTP status code in the 4xx class.

A 404 error (404 Not Found) is the HTTP status code a server returns when it finds nothing at the requested URL.

410 Gone is an HTTP status code that says a resource is no longer available on the server and that this is likely to be permanent.

The HTTP status code 429 Too Many Requests means a client has sent too many requests in a given time.

451 Unavailable For Legal Reasons is an HTTP status code that signals the server denies access to a resource because of a legal demand.

A 500 Internal Server Error is a generic HTTP server error.

The HTTP status code 502 Bad Gateway means a server acting as a gateway or proxy received an invalid response from an upstream server …

The HTTP status code 503 Service Unavailable means the server is not ready to handle the request.

The HTTP status code 504 Gateway Timeout means a server acting as a gateway or proxy did not receive a timely response from the upstream …

A broken link is a hyperlink whose target cannot be loaded, usually because the target page returns an error such as 404 or does not …

A custom 404 page is the page a server shows when a requested URL does not exist, designed to help visitors continue.

Dead-end pages are pages where a visitor has no obvious next step, because no link or call to action leads onward.

An HTTP status code is the three-digit number a web server sends with every response.

A 301 redirect is a server response with the HTTP status code 301 Moved Permanently.

A 302 redirect (status code 302 Found) tells browsers and crawlers that a resource is temporarily available at a different URL, given in …

A 307 redirect (Temporary Redirect) is an HTTP status code that sends a client to another URL for a limited time.

A 308 redirect (Permanent Redirect) is an HTTP status code for a permanent move.

A JavaScript redirect is a redirect that the browser carries out with a script, usually by setting window.location.

Meta refresh is an HTML <meta http-equiv="refresh"> tag that tells the browser to reload the page or load another URL after a set number …

A redirect sends browsers and crawlers that request one URL automatically to a different URL.

A redirect chain is a sequence of two or more redirects between the requested URL and the final page, for example A to B to C. A single …

A redirect loop exists when URLs redirect to each other in a circle, for example A to B and B back to A. The target page is never reached …

A ccTLD (country code top-level domain) is a top-level domain with two letters that is assigned to a country or territory, such as .de for …

A domain name is a text label such as example.com that identifies a host on the internet and maps, through the Domain Name System (DNS) …

A dynamic URL contains parameters that the server evaluates before it returns the page, for example /product?id=12&color=red.

A fragment identifier is the last part of a URL, starting with the # character.

A mirror site is a replica of a website hosted on a different server under a different URL.

The query string is the part of a URL that starts with a question mark (?) and carries additional data as key and value pairs.

An SEO-friendly URL (also called a clean or readable URL) is an address that uses readable words to describe the page, so people and …

A session ID in a URL is an identifier that links requests to one visitor session and is appended to the address, for example ?sid=a1b2c3.

A static URL is an address without parameters, for example /shoes/sneakers.

A subdirectory (also called a subfolder) is a section of a website that sits in the path of the same domain, for example example.com/blog/.

A subdomain is a name placed in front of a domain, separated by a dot, for example blog in blog.example.com.

A top-level domain (TLD) is the last label of a domain name, the part after the final dot, for example com in example.com.

A trailing slash is the forward slash at the end of a URL, as in https://example.com/foo/.

A URL (Uniform Resource Locator) is the address of a unique resource on the web, such as an HTML page, an image or a document.

URL case sensitivity describes whether uppercase and lowercase letters in a URL count as different.

URL hijacking is another name for typosquatting: registering domains that differ from a well-known brand's address only by a typing …

URL parameters are key and value pairs added to a URL after a question mark.

URL structure is the way a website builds its addresses: protocol, host, path, and optional parameters.

UTM parameters are tags added to the end of a URL, such as utm_source and utm_medium, that tell an analytics tool which campaign, channel …

A vanity URL is a short, memorable web address that leads to a longer or more technical destination URL.

WHOIS is a protocol and service for looking up registration data about domains and IP addresses, such as the registrar, name servers and …

www vs. non-www is the choice between www.example.com and example.com as the main address of a website.

Browser caching (also called HTTP caching) means that a browser stores a copy of a response, such as an image, stylesheet or script, and …

Cache-Control is an HTTP header that tells browsers and shared caches such as CDNs whether and for how long they may store a response.

A CDN (content delivery network) is a geographically distributed network of servers that stores copies of a website's files and delivers …

An ETag (entity tag) is an HTTP response header that identifies a specific version of a resource.

HSTS (HTTP Strict Transport Security) is a security mechanism that tells browsers to access a host only over HTTPS for a set period.

.htaccess is a plain-text configuration file for the Apache web server.

HTTP/2 is the second major version of the HTTP protocol, standardized in RFC 9113 (the current revision, which replaces RFC 7540).

HTTP/3 is the third major version of the HTTP protocol, standardized as RFC 9114 in June 2022.

An HTTP header is a name-value pair that a client or server sends along with an HTTP request or response to pass extra information, such …

The HTTP Link header is a response header that expresses links between resources, with the same meaning as the HTML <link> element.

HTTPS (Hypertext Transfer Protocol Secure) is the HTTP protocol sent over an encrypted TLS connection.

An IP address is a numeric label that identifies a device or server in a network that uses the Internet Protocol, and it tells other …

IPv6 (Internet Protocol version 6) is the successor of IPv4 and is specified in RFC 8200.

The Last-Modified header is an HTTP response header with the date and time the server believes a resource was last changed.

Mixed content occurs when a page loaded over HTTPS includes resources, such as images, scripts or stylesheets, that are fetched over …

A reverse DNS lookup (rDNS) is a DNS query that finds the hostname belonging to an IP address.

Server configuration, in an SEO context, means the settings of a web server such as nginx or Apache that decide how requests are answered.

Server response time is the time a server needs to answer a request.

An SSL/TLS certificate is a digital file that binds a public key to a domain name, so that a browser can check it is talking to the right …

The Vary header is an HTTP response header that lists which request headers influenced the response, so that caches store and serve each …

A web application firewall (WAF) inspects the HTTP and HTTPS traffic that reaches a website or web application and blocks suspicious …

Web hosting is the infrastructure that serves a website.

AMP (Accelerated Mobile Pages) is an open-source framework for fast-loading web pages, announced by Google in 2015.

The anchor element <a>, often called anchor tag, creates a hyperlink in HTML.

The BFSG (Barrierefreiheitsstärkungsgesetz, German Accessibility Act) of July 16, 2021 sets accessibility requirements for certain …

Cloaking in SEO means serving search engine crawlers different content than human visitors under the same URL.

Crawlable links are links that a crawler can follow reliably.

Dynamic serving is a mobile configuration in which one URL returns different HTML depending on the device.

HTML (HyperText Markup Language) is the markup language that defines the meaning and structure of web content.

The HTML head, written as the <head> element, holds machine-readable metadata about a document.

HTML validation, often called W3C validation, checks whether a page's markup follows the rules of the HTML specification.

An iframe (inline frame) is an HTML element that embeds another HTML document in the current page, for example a video player, a map or a …

indexifembedded is a robots rule that lets Google index the content of a page when that content is embedded in another page through an …

Infinite scroll is a design pattern where new content loads automatically as a visitor scrolls down, so there is no click to a second page.

max-image-preview is a robots rule that sets the largest size in which Google may show an image from a page as a preview.

Mobile-friendliness describes how easy a website is to read and use on a smartphone.

Pagination on the web splits a long list across several numbered pages, such as the products of a store category, blog archives, or search …

A progressive web app (PWA) is an app built with web technologies that can be installed and used like a platform-specific app, partly even …

rel=next and rel=prev are link relations that mark a page as part of a series and point to the following and preceding page, for example …

Responsive design is a web design approach in which one URL and one HTML document serve every device, while CSS adapts the layout to the …

Semantic HTML means choosing HTML elements by what they mean instead of how they look.

Separate mobile URLs are a mobile setup where desktop and phone visitors receive different HTML at different URLs, usually on a subdomain …

The viewport meta tag (<meta name="viewport">) gives browsers hints about how to size and scale the visible area of a page, mainly on …

The WCAG (Web Content Accessibility Guidelines) are the W3C's technical standard for accessible web content.

Web accessibility means that websites, tools and technologies are designed so that people with disabilities can use them.

XML (Extensible Markup Language) is a text-based format that describes data in nested elements with self-defined tags.

ads.txt (Authorized Digital Sellers) is a public text file that lets publishers declare which companies may sell their digital ad inventory.

A CMS (content management system) is software for creating, editing, managing and publishing digital content, usually website content …

A favicon (favorite icon) is the small icon of a website.

A hacked site is a website on which third parties have placed content, code or redirects without permission.

A headless CMS is a content management system that stores and manages content and delivers it through an API, without producing web pages …

humans.txt is a text file in the root of a website that names the people who built it, such as developers, designers and writers.

Maintenance mode (SEO meaning: a website temporarily shows a notice page while work is done) should answer with HTTP status 503 Service …

Password protection, in the website sense, means locking a site or directory behind a username and password, most often with HTTP Basic …

security.txt is a plain text file in which an organization states how to report security vulnerabilities on its website.

A staging environment (also called a dev or test environment) is a copy of a website or application where changes are tested before they …