Final answer:
Stiff peak egg whites are firm and hold their shape when beaten.
Step-by-step explanation:
Stiff peak egg whites refer to the stage in beating egg whites where they become firm and hold their shape.
When you beat egg whites, the proteins in the egg whites unfold and stretch out, forming a network that traps air bubbles. As you continue to beat, the proteins firm up and stabilize the air bubbles, resulting in an increase in volume and the formation of stiff peaks.
An easy way to test if you have achieved stiff peaks is to lift the beaters from the egg whites, and if the peaks that form hold their shape without drooping, then you have stiff peaks.