A mockbuster (sometimes also called a knockbuster or a drafting opportunity) is a film created with the intention of piggy-backing on the publicity of a major film with a similar title or theme. Mockbusters are often made with a low budget. These films are usually released direct-to-video at the same time as the mainstream film reaches theaters or video outlets.
It is possible to use these kinds of movies to intentionally deceive consumers into mistakenly purchasing the derivative title (e.g., someone thinks they are buying a copy of Transformers, but they are actually getting Transmorphers), but another possibility is to offer legitimate add-on buying opportunity in the marketplace (e.g., customer enjoyed Will Ferrell's Land of the Lost and wants more in the same subgenre, and buys or rents C. Thomas Howell's The Land That Time Forgot).
Mockbusters have a long history in Hollywood and elsewhere. For example, the 1959 Vanwick film The Monster of Piedras Blancas was a clear derivative of Creature from the Black Lagoon, complete with a creature suit by the same designer, Jack Kevan. Attack of the 50 Foot Woman spawned Village of the Giants; The Land That Time Forgot spun Legend of Dinosaurs & Monster Birds; Star Wars gave derivative birth to a jumble of imitations — Starcrash and Battle Beyond the Stars among others. The success of Steven Spielberg's 1982 family-film E.T. the Extra-Terrestrial spawned the infamous 1988 film Mac and Me, which was critically panned for capitalizing on E.T.'s popularity and for its extensive use of product placement. The 1984 film Gremlins and its ensuing popularity inspired the creation of the notoriously bad 1988 ripoff Hobgoblins. The success of Raiders of the Lost Ark spawned a number of adventure films and TV series taking place in the 1930s and 1940s, including King Solomon's Mines, High Road to China, Tales of the Gold Monkey, and Bring 'Em Back Alive.