In film parlance, a sequence is a series of scenes which comprise a distinct narrative unit, usually connected either by unity of location or unity of time. For example, in a heist film, there might well be an extended recruitment sequence where the leader of the gang collects together his or her co-conspirators, a robbery sequence, an escape sequence, and so on. Each of these sequences might well contain sub-sequences; for example the robbery sequence might consist of an entry sequence, a safe-cracking seqence, and so on.
The sequence is just one structural unit in the hierarchy which is used to describe the structure of films in varying degrees of granularity. Analysed this way, a film is comprised of a one or more acts; acts are comprised of a one or more sequences; sequences are comprised of one or more scenes, and scenes may be thought of as being built out of shots (if one is thinking visually) or beats (if one is thinking in narrative terms).
See also:
