The term B movie originally referred to a motion picture made on a low or modest budget and intended for distribution as the less-publicized, bottom half of a double feature during the so-called Golden Age of Hollywood. Although the U.S. production of movies intended as second features largely ceased by the end of the 1950s, the term B movie continues to be used in a broader sense, referring to any low-budget, commercial motion picture meant neither as an arthouse film nor as pornography. In its post Golden Age usage, there is ambiguity on both sides: on the one hand, many B movies display a high degree of craft and aesthetic ingenuity; on the other, the primary interest of many inexpensive exploitation movies is prurient. In some cases, both are true.

In either usage, most B movies represent a particular genre. The Western was a Golden Age B movie staple, while low-budget science-fiction and horror films became more popular in the 1950s. Early B movies were often part of series in which the star repeatedly played the same character. Almost always shorter than the top-billed films they were paired with, many had running times of 70 minutes or less. The term connoted a general perception that B movies were inferior to the more handsomely budgeted headliners; individual B films were often ignored by critics. Latter-day B movies still sometimes inspire multiple sequels, but series are less common. As the average running time of top-of-the-line films increased, so did that of B pictures. In its current usage, the term has two primary and somewhat contradictory connotations: it may signal an opinion that a certain movie is (a) a genre film with minimal artistic ambitions or (b) a lively, energetic film uninhibited by the constraints imposed on more expensive projects and unburdened by the conventions of putatively "serious" independent film.