An X-certificate was a rating given to films in the UK, denoting that they were not suitable for children under 18 years of age. However, as this rating is no longer used, it may be useful to know other synonyms for X-certificate. Some alternatives might include "Adults Only," "Mature Audience," or "Restricted" depending on the context in which the rating is used. Other possibilities might be "18+" or "Explicit Content". These synonyms can help provide a better understanding of the content and the audience appropriateness for different materials. Ultimately, the goal of any rating system is to help people make informed decisions about what they watch or consume.