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A Mutual as defined by Urban Dictionary is:
Internet persona(s) who happens to follow you and you happen to follow them back and maintain communication with and have a mutual liking for eachother whether it be friendship wise, etc.
In fandom terms this usually means fellow fans that have the same fandom interests that you do. The term is most often seen on platforms such as Tumblr or Twitter. Mutuals follow each other's blogs or social media feeds and like and comment on each other's posts, as well as having fannish discussions and occasionally working on fannish projects together, sometimes mutuals will become online or IRL friends.