a low-budget movie made especially to accompany a major feature film on a double bill.
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for b movie
a film originally made (esp in Hollywood in the 1940s and 50s) as a supporting film, now often considered as a genre in its own right
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin and History for b movie
by 1939, usually said to be so called from being the second, or supporting, film in a double feature. Some film industry sources say it was so called for being the second of the two films major studios generally made in a year, and the one cast with less headline talent and released with less promotion. And early usage varies with grade-B movie, suggesting a perceived association with quality.
Online Etymology Dictionary, © 2010 Douglas Harper