The 18 certificate is issued by the British Board of Film Classification (BBFC), to state that in its opinion, a film, video recording, or game should not be seen in a cinema or purchased by a person under 18 years old.
As with other British film certificates, the 18 certificate theoretically only has advisory power for films shown in public cinemas, with the ultimate say being held by local authorities. In practice, the local authorities tend to follow BBFC rulings in all but a few exceptional cases.
For video and game sales, the BBFC rulings have statutory power, as under the terms of the 1984 Video Recordings Act all videos sold or distributed within the UK must be given a certificate by the BBFC, unless they fall into one of a number of exempt categories. Uncertificated recordings which are not exempt cannot legally be sold, regardless of content.
The 18 certificate was created in 1982 as the successor of the previous X certificate, which in turn was the successor of the H certificate (with H standing for "horror"). See History of British Film Certificates for more details.
Typical reasons for restricting films to the 18 certificate category have included scenes of hard drug use, supernatural horror, explicit sex, sadistic violence and sexual violence — the latter two of which have in the past led to a certificate not being issued at all, in effect banning the film in the UK.
It was only around the start of the 21st century that the censors passed films with explicit ('hardcore') sexual acts despite the 18 certificate existing for many years.
It is also issued by the Irish Film Classification Office.
Until recently, 18 certificate films could not contain the depiction of actual sex acts, which more recently could only appear in films with an R18 certificate (when created in 1982 only simulated acts could be shown under an R18, but this was relaxed after legal challenges in 2000). Although the BBFC allowed the depiction of simulated sex scenes in 18 certificate films, actual sexual acts were still not allowed to be depicted in 18 certificate films until around 1999.