|Synonyms:||R, M, Adult|
|See also:||Rating, Teen, Het, Slash, Gen|
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It usually means the fanworks have non-explicit adult themes (such as some sexual content), references to violence, and strong coarse language. Archive of Our Own defines its "Mature" rating to indicate: "This is for content with adult themes (sex, violence, etc.) that isn't as graphic as explicit-rated content."
The rating is roughly equivalent to an MPAA "R" (Restricted) rating.