Featurette is a term used in the American film industry to designate a film whose length is approximately three quarters of a reel, or about 20–44 minutes in running time - thus midway between a short subject and a feature film; thus it is a "small feature" (the ending "-ette" is a common diminutive suffix derived from French). The term was commonly used from before the start of the sound era into the 1960s, when films of such length such as the Hal Roach's Streamliners and several French films of that length ceased being made or were made as experimental or art films and subsumed under the more general rubric of short. Its use outside the USA is unknown, although it was as commonly applied to foreign imports as domestic productions within that country.
After the advent of DVD, the term - at least in America - also gained the meaning of "a brief documentary film covering one or more aspects of the film creation process". Featurettes are usually in DVD features descriptions to refer to video-format "Behind The Scenes" type bonus features such as documentaries on special effects or set design or cast and crew interviews. This usage possibly derived partly or completely independent from the term's original meaning, as should be apparent from the fact that most Blu-ray "featurettes" are actually less (sometimes far less) than 20 minutes in length.
Today, the latter definition, though possibly an inaccurate usage of the original term, is far more common.