Definition: beating butter and sugar together until combined and lightened, to build texture and — depending on the bake — trap air.
Creaming is the quiet workhorse of baking. For a fluffy sponge you cream well, 3–4 minutes, until pale and airy, because that trapped air is your rise. For cookies it’s the opposite instinct: creamjust until combined and a shade lighter, because too much air makes them spread flat. Same word, two goals — knowing which you’re after is half the battle.
In practice: soft (not melted) butter at room temperature creams best. Pale and fluffy for cakes; barely combined for chewy cookies.