A semidocumentary is a form of book, film, or television program presenting a fictional story that incorporates many factual details or actual events, or which is presented in a manner similar to a documentary.
Stylistically, it has certain similarities to Italian Neorealism, such as the use of location shooting and employing non-actors in secondary roles.
However, the viewer is not intended to mistake a semidocumentary for a real documentary; the fictional elements are too prominent.
[1] One of the first films of this kind was Henry Hathaway's The House on 92nd Street (1945).
The standard documentary had blurred the difference between itself and fiction so much that there was viewer confusion regarding what they were seeing.