Plain English Meaning
A setup where the two ailerons don't move the same amount when you turn the control wheel. The one that goes up moves further than the one that goes down. This helps the airplane turn cleanly instead of swinging its nose the wrong way at the start of a turn.
Full Definition
An aileron design in which the up-going aileron deflects a greater distance than the down-going aileron on the opposite wing. This unequal travel reduces adverse yaw by increasing drag on the wing being lowered (rising aileron) and producing less drag on the wing being raised (lowered aileron).
Context Anchor
Seen when studying airplane control surfaces and how the airplane responds to a turn input from the control wheel or stick.
Derivation
‘Differential’ comes from Latin differentia, meaning ‘a difference.’ Here it simply means the two ailerons move by different amounts — a built-in difference in their travel.