Action economy
Also said as Actions per turn, Tempo
In a turn-based fight, how many things each side gets to do. A party of four with four actions usually beats one monster with four times the health and damage, because that single monster wastes damage and can be stunned out of its whole turn.
It is why boss fights that look fair on paper end in two rounds. It is also why a stun is so strong, and why anything giving an extra action tends to be the best thing in the game.
Say it
“The solo boss dies in two rounds, because it acts once while the party acts four times. Give it three small actions a round, in place of one big one, and keep the total damage the same.”
Where you’ve seen it
Dungeons & Dragons 5th edition gives powerful solo monsters legendary actions, taken at the end of other creatures' turns, so that one creature isn't simply out-acted by a party.
Not to be confused with dps
DPS adds up total damage. Action economy is about how that damage is divided into turns, which decides how much of it is wasted or lost to one bad moment.