Plain English Meaning
A wing shaped like a stretched oval when viewed from above. It is very efficient in the air, but it can stall all at once with little warning, and it is harder to build.
Full Definition
A wing planform whose leading and trailing edges form the shape of a half-ellipse, producing a lift distribution along the span that is closest to the theoretical ideal. This shape minimizes induced drag for a given span and lift, but it tends to stall along the entire span at nearly the same time, giving little stall warning. It is also more difficult and expensive to manufacture than straight or tapered wings.
Context Anchor
Seen in discussions of wing planform, aircraft performance, lift distribution, and induced drag.
Derivation
From the geometric shape called an ellipse — an oval, symmetrical curve. The wing is named after this shape because, viewed from above, its outline traces a half-ellipse.