A directorial beat is a unit of script analysis representing the smallest defined action in a play script, typically an exchange of behaviours between characters in a script. It usually takes the form of action-reaction. Each scene beat by beat, with the characters progressing the action in this, the smallest element of story structure.
The directorial beat should not be confused with a screenplay beat, which indicates a short pause.
References
Directorial beat Wikipedia(Text) CC BY-SA