Cosplay, a blend word of "costume play", is an activity and performance art in which participants called cosplayers wear costumes and fashion accessories to represent a specific character. Cosplayers often interact to create a subculture, and a broader use of the term "cosplay" applies to any costumed role-playing in venues apart from the stage. Any entity that lends itself to dramatic interpretation may be taken up as a subject. Favorite sources include anime, cartoons, manga, comic books, television series, rock music performances, video games, memes, and in some cases, original characters. The term has been adopted as slang, often in politics, to mean someone pretending to play a role or take on a personality disingenuously.

Cosplay grew out of the practice of fan costuming at science fiction conventions, beginning with Morojo's "futuristicostumes" created for the 1st World Science Fiction Convention held in New York City in 1939. The Japanese term "cosplay" (コスプレ, kosupure) was coined in 1983. A rapid growth in the number of people cosplaying as a hobby since the 1990s has made the phenomenon a significant aspect of popular culture in Japan, as well as in other parts of East Asia and in the Western world. Cosplay events are common features of fan conventions, and today there are many dedicated conventions and competitions, as well as social networks, websites, and other forms of media centered on cosplay activities. Cosplay is very popular among all genders, and it is not unusual to see crossplay, also referred to as gender-bending.

The term "cosplay" is a Japanese blend word of the English terms costume and play. The term was coined by Nobuyuki Takahashi [ja] of Studio Hard in an article for the Japanese magazine My Anime [ja] in June 1983. Takahashi decided to coin a new word rather than use the existing translation of the English term "masquerade" because it implied nobility and was old-fashioned. The coinage reflects a common Japanese method of abbreviation in which the first two moras of a pair of words are used to form an independent compound: 'costume' becomes kosu (コス) and 'play' becomes pure (プレ).

Masquerade balls were a feature of the Carnival season in the 15th century, and involved increasingly elaborate allegorical Royal Entries, pageants, and triumphal processions celebrating marriages and other dynastic events of late medieval court life. They were extended into costumed public festivities in Italy during the 16th century Renaissance, generally elaborate dances held for members of the upper classes, which were particularly popular in Venice.

In April 1877, Jules Verne sent out almost 700 invitations for an elaborate costume ball, where several of the guests showed up dressed as characters from Verne's novels.

Costume parties (American English) or fancy dress parties (British English) were popular from the 19th century onwards. Costuming guides of the period, such as Samuel Miller's Male Character Costumes (1884) or Ardern Holt's Fancy Dresses Described (1887), feature mostly generic costumes, whether that be period costumes, national costumes, objects or abstract concepts such as "Autumn" or "Night". Most specific costumes described therein are for historical figures although some are sourced from fiction, like The Three Musketeers or Shakespeare characters.

By March 1891, a literal call by one Herbert Tibbits for what would today be described as "cosplayers" was advertised for an event held from 5–10 March that year at the Royal Albert Hall in London, for the so-named Vril-Ya Bazaar and Fete based on a science fiction novel and its characters, published two decades earlier.

A.D. Condo's science fiction comic strip character Mr. Skygack, from Mars (a Martian ethnographer who comically misunderstands many Earthly affairs) is arguably the first fictional character that people emulated by wearing costumes, as in 1908 Mr. and Mrs. William Fell of Cincinnati, Ohio, are reported to have attended a masquerade at a skating rink wearing Mr. Skygack and Miss Dillpickles costumes. Later, in 1910, an unnamed woman won first prize at masquerade ball in Tacoma, Washington, wearing another Skygack costume.