Verb
Also said as Player action, What the player can do
What the player can do, named as a doing word: run, jump, shoot, trade, talk. The list of verbs is the quickest honest description of any game.
Two games with the same art and story but different verbs are different games. If you cannot list the verbs, you do not yet know what you are making, and nor does anyone you ask to build it.
Say it
“This game has three verbs: run, jump and grapple. Anything that needs a fourth button gets cut, or becomes a new use of those three.”
Where you’ve seen it
Mario began as Jumpman in Donkey Kong, a character named after his verb. VVVVVV takes the jump away altogether: besides walking, your only verb is flipping gravity, and the whole game grows out of that.
Not to be confused with mechanic
A verb is what the player does. A mechanic is the whole rule behind it, including what the game does back.