n. — abbreviation
a movie rating to restrict viewing to those aged 18 and older, unless accompanied by an adult.
Type: 4. Culturally Significant — A film marked 18A is considered suitable for viewers 18 years of age or older; those who are under 18 must be accompanied by an adult, hence the term 18 Accompaniment or 18A. An 18A movie will likely involve coarse language, sexual activity and explicit violence.
Note that the definition of 18A is only standardized for home videos and not for theatrical releases of motion pictures; thus, theatrical releases' assessment can vary provincially. For example, Manitoba and the Maritimes restrict viewers that are accompanied by an adult to 14 or older, whereas Alberta and Ontario allow all ages under 18 (see Gov't of Manitoba reference). In the home-video context, classification is determined based on an average of provincial Film Review Board ratings that are calculated by Canada's Motion Picture Association and standardized by the voluntary Canadian Home Video Rating System, or CHVRS (see Motion Picture Association reference).
The rating 18A differs from the Canadian "R", or Restricted rating, in which no persons under 18 may attend. The rating 18A in Canada is "roughly equivalent" to the R rating in the US (see first 1999 quotation), which allows underage viewing with adult accompaniment.