Conventional signs
Glossary
Short definitions of the terms used in our reports and guides.
A
- A record
- A DNS record that holds an IPv4 address for a name, telling browsers and other software which server to connect to.
- AAAA record
- A DNS record that holds an IPv6 address (a 128-bit address) for a name. It is the IPv6 equivalent of an A record and is defined in RFC 3596.
- ASN
- Autonomous System Number: the number identifying an autonomous system, a group of IP networks run by one or more network operators with a single, clearly defined routing policy. Looking up the ASN for a website's IP address shows which network it is hosted on.
C
- CAA record
- Certification Authority Authorization: a DNS record that lets a domain holder list which certificate authorities may issue certificates for the domain, reducing the risk of a certificate being issued by mistake.
- Canonical URL
- The preferred address for a page that is available at several addresses with the same or very similar content. Site owners can declare it with a link element using rel="canonical" (RFC 6596).
- ccTLD
- Country-code top-level domain: a TLD associated with a country or territory, such as .uk or .fr. Each one sets its own rules; .uk is run by Nominet.
- Certificate authority
- An organisation that issues and signs digital certificates, vouching that a public key belongs to the named domain or organisation. A domain's CAA record can limit which certificate authorities may issue certificates for it.
- CNAME record
- A DNS record that makes one name an alias of another, the canonical name. Software looking up the alias is pointed to the target name's records instead.
- Common Crawl
- A non-profit founded in 2007 that maintains a free, open repository of web crawl data that anyone can use. It also publishes host-level and domain-level web graphs.
D
- DKIM
- DomainKeys Identified Mail (RFC 6376): a method of adding a cryptographic signature to email so receivers can confirm that a domain takes responsibility for a message. The public key used to check the signature is published in the domain's DNS.
- DMARC
- Domain-based Message Authentication, Reporting and Conformance: a TXT record published at _dmarc.example.co.uk that tells receiving mail servers how the domain owner wants mail that fails DMARC validation (which builds on SPF and DKIM) to be handled, and where to send reports. The current specification is RFC 9989 (May 2026), which replaced RFC 7489.
- DNS
- The Domain Name System: the distributed system that translates names into IP addresses and other data. A resolver finds the answer by following referrals from the root to the TLD and on to the domain's own nameservers, and caches results to speed things up.
- DNSSEC
- DNS Security Extensions: digital signatures added to DNS data so that resolvers can check the answer really came from the zone's owner and was not altered on the way. DNSSEC protects integrity; it does not encrypt queries.
- Domain name
- A human-readable name such as example.co.uk, made of labels separated by dots, that the Domain Name System (DNS) translates into the information computers need, such as IP addresses. Each label can contain letters, numbers and hyphens, and can be up to 63 characters long.
- Drop-catching
- Registering a domain the moment it is released ("drops") after deletion, usually by setting up in advance. For .uk, Nominet publishes a daily drop list giving the exact time each name will become available.
E
- EPP status codes
- Short codes showing what state a domain is in, such as ok, clientTransferProhibited, redemptionPeriod or pendingDelete. Codes starting 'client' are set by the registrar and codes starting 'server' by the registry; clientTransferProhibited, for example, makes the registry reject transfers to another registrar until it is removed.
G
- gTLD
- Generic top-level domain: a TLD that is not tied to a country, such as .com, .org or .app. Registries and registrars for gTLDs must follow ICANN policies, including its rules on expiry and redemption.
H
- HSTS
- HTTP Strict Transport Security (RFC 6797): a Strict-Transport-Security header with which a website tells browsers to connect to it only over secure HTTPS connections in future.
- HTTP status code
- The three-digit number a web server returns with every response. The first digit gives the class: 1xx informational, 2xx success (such as 200 OK), 3xx redirection, 4xx client error (such as 404 Not Found) and 5xx server error.
I
- IDN
- Internationalised domain name: a domain name containing characters beyond the basic a-z, 0-9 and hyphen, such as accented letters or non-Latin scripts. In the DNS itself an IDN is stored in an ASCII form that begins xn--.
- IPv6
- Internet Protocol version 6, the successor to IPv4. It increases the size of an IP address from 32 bits to 128 bits, giving vastly more addresses.
M
- Meta robots / noindex
- A robots meta tag in a page's HTML head (or the equivalent X-Robots-Tag HTTP header) that gives search engines page-level instructions. The noindex rule asks them not to show the page in search results.
- MX record
- A DNS record naming a mail server that accepts email for a domain, with a preference number; lower numbers are preferred.
N
- Nameserver
- A server that answers DNS queries for a domain. Each domain is served by one or more nameservers, which hold its records and tell the world where its website and email are.
- NS record
- A DNS record that lists the authoritative nameservers for a zone. The NS records held by the parent zone, such as .uk, are what delegate a domain to its nameservers.
O
- Open Graph
- A set of meta tags (the Open Graph protocol) that describe a page when it is shared on social media. The four required properties are og:title, og:type, og:image and og:url.
P
- Public Suffix List
- A community-maintained list, started by Mozilla, of suffixes under which people can register names, such as com and co.uk. Browsers and other software use it to work out the registrable part of a domain, for example to stop 'supercookies' being set on a whole suffix such as co.uk.
- Punycode
- The encoding, defined in RFC 3492, that turns a Unicode label into ASCII letters, digits and hyphens so that internationalised domain names can be stored in the DNS. A label converted this way carries the prefix xn--.
R
- RDAP
- Registration Data Access Protocol: the modern way to look up a domain's registration data, such as its registrar, key dates and status codes, returned as structured JSON. ICANN made RDAP the definitive source for gTLD registration data from 28 January 2025, and Nominet provides RDAP for .uk.
- Redemption grace period
- A period after a domain has been deleted or suspended for non-renewal during which only the previous registrant can restore it. For gTLDs it lasts 30 days after deletion; for .uk it currently runs from 30 to 90 days after expiry, and is due to become 30 days when Nominet moves .uk to its new lifecycle on 9 February 2027.
- Redirect
- A 3xx HTTP response telling the browser to fetch a different address, given in the Location header. 301 (Moved Permanently) and 308 (Permanent Redirect) signal a permanent move; 302 (Found) and 307 (Temporary Redirect) a temporary one.
- Registrable domain
- The part of a name that someone can actually register: the public suffix plus the one label in front of it. For www.shop.example.co.uk the public suffix is co.uk, so the registrable domain is example.co.uk.
- Registrant
- The person or organisation on whose behalf a domain name is registered: in everyday terms, the domain's holder.
- Registrar
- A service provider that acts as the go-between for registrants and registries, selling and managing domain registrations for its customers. For .uk names, registrars work with Nominet.
- Registry
- The organisation that runs a zone such as a TLD and decides what can be registered in it, keeping the central database of registrations. Nominet is the registry for .uk.
- robots.txt
- A plain-text file at the root of a website that tells automated crawlers which parts of the site they may access, under the Robots Exclusion Protocol (RFC 9309). The rules are a request to well-behaved crawlers, not a form of access control.
S
- SAN
- Subject Alternative Name: the part of a certificate that lists the domain names (and other identities) it is valid for. One certificate can cover several names, such as example.co.uk and www.example.co.uk.
- Second-level domain
- The label immediately to the left of the TLD. In the .uk space, zones such as co.uk and org.uk are second-level domains under which people register names like example.co.uk; since 2014 people can also register directly at the second level, as in example.uk.
- SOA record
- Start of Authority: the record that marks the start of a DNS zone. It names the primary nameserver and a contact mailbox and holds a serial number and timers used when copying the zone between servers.
- SPF
- Sender Policy Framework (RFC 7208): a TXT record in which a domain lists the servers allowed to send email using that domain, so receiving servers can check messages against it.
- SSL/TLS certificate
- A digital certificate that binds a domain name to a public key and is signed by a certificate authority, so a browser can set up an encrypted TLS connection and check it has reached the right site. SSL is TLS's older predecessor and is now deprecated, but 'SSL certificate' remains the everyday name.
- Subdomain
- A domain that sits inside another one: shop.example.co.uk is a subdomain of example.co.uk. Whoever controls a domain can create subdomains beneath it without registering anything new.
T
- TLD
- Top-level domain: the last label of a domain name, one layer below the root of the DNS, such as com or uk. TLDs are commonly grouped into country-code TLDs (ccTLDs) and generic TLDs (gTLDs).
- Tranco list
- A research-oriented ranking of the top one million domains, built by averaging several popularity rankings over the past 30 days. It was designed to be more stable and harder to manipulate than any single ranking.
- TTL
- Time to live: the number of seconds a DNS record may be cached before it should be looked up again. Long TTLs make changes slower to spread; short ones mean more lookups.
- TXT record
- A DNS record that holds free-form text strings. It has many uses; email authentication policies such as SPF and DMARC, for example, are published as TXT records.
W
- Wayback Machine
- The Internet Archive's free service for browsing archived copies of web pages, opened to the public in 2001 from web crawls dating back to 1996. Its earliest capture of a domain is a useful clue to the domain's history.
- WHOIS
- The older, plain-text protocol (RFC 3912) for querying domain registration databases; the word is still widely used for registration data in general. It is being replaced by RDAP, and Nominet says .uk will stop using WHOIS on 9 February 2027.
X
- XML sitemap
- An XML file listing a site's URLs, optionally with details such as when each was last changed, so search engines can find and crawl its pages more intelligently. The format is described at sitemaps.org.