Beat

A beat is the smallest unit of change in a story — one action, reaction or shift in intention — and, in prose, also the small piece of stage business used in place of a dialogue tag.

The word carries two working meanings, and writers meet both.

The structural beat, borrowed from screenwriting, is the smallest exchange in which something changes. A scene is a sequence of beats; each one ends when a character gets what they wanted, is refused, or changes what they want. A beat sheet is simply that list of changes laid out across the whole story, which is why it is a faster planning tool than a chapter outline — it tracks movement rather than events.

The action beat is the piece of behaviour that replaces an attribution. "I'm fine." She did not look up from the sink. It tells you who is speaking, it controls the pace by putting a pause where you want one, and it characterises — three jobs that she said dismissively cannot do at once. See dialogue tag.

In a screenplay you will also meet (beat) on its own line, which is a direction for a pause in delivery.