Beat

The smallest unit of story movement — one shift in a scene, a character, or the plot's direction.

A beat is one unit of change: a reversal, a decision, a reveal, a shift in who holds power in a scene. Writers use the word at two scales — the micro-beat inside a scene (an actor's change of tactic) and the structural beat (a landmark like the midpoint that a whole act pivots on).

Planning at the beat level is what separates outlining from summarizing: a summary says what happens, a beat sheet says what changes.