Filk music is a musical culture, genre, and community tied to science fiction/fantasy fandom and a type of fan labor. The genre has been active since the early 1950s, and played primarily since the mid-1970s. The term (originally a typographical error) predates 1955.
As Interfilk's What it is? page demonstrates, there is no consensus on the definition of filk. Attempts have been made to define filk based on various criteria.
Filk has been defined as folk music, usually with a science fiction or fantasy theme, but this definition is not exact. Filkers have been known to write filk songs about a variety of topics, including but not limited to tangentially related topics such as computers and cats. In addition, while the majority of filk songs are in the folk style, other styles such as blues, calypso, and even rock appear from time to time.
Filk has been defined as what is sung or performed by the network of people who originally gathered to sing at science fiction/fantasy conventions. This definition might be summarized as, "We can argue what it is until we're red or green or blue, but when filkers get together it's the thing we do."