Intensity curve
Also said as Drama curve, Encounter pacing
How intense the action feels from moment to moment, drawn as a line. Designers shape it with peaks and valleys, and some games measure it live and react.
Intensity is what players will remember as the shape of the game. A flat line feels dull even when it is dangerous, and a line that never falls leaves people numb.
Say it
“Drive enemy spawns from a director: build until player intensity reaches 80%, hold for four seconds, then send nothing for at least ten seconds, whatever the level says.”
Where you’ve seen it
Left 4 Dead's AI Director tracks each survivor's intensity. It builds pressure to a peak, holds it for a short time, then backs off into a real lull. Valve's Mike Booth called this adaptive dramatic pacing.
Not to be confused with difficulty curve
Difficulty is how hard it is to succeed. Intensity is how much is going on. A swarm of weak enemies can be intense and easy.