Japan is generally credited as the origin of cosplay, but there are no actual facts of when the cosplay culture started. The credit for coining the word ‘cosplay’ seems more apparent though… although there are a few versions on how it was created, the credit goes to Nobuyuki Takahashi when he first used the word in some Japanese magazines in 1983/1984.

Akibanana has some scanned image of the My Anime magazine released in June 1983, which is said to be where the word first appeared, written in Japanese – コスプレ (kosupure)…

Origin of the word cosplay from YeinJee's Asian Blog