Plain English Meaning
The shape of a wing when you look straight down at it from above. It tells you whether the wing is straight, swept back, tapered, rectangular, or some other shape.
Full Definition
The shape and outline of a wing as viewed from directly above or below. Planform describes characteristics such as wingspan, chord, taper, sweep, and overall geometric shape, all of which influence the wing's aerodynamic behavior, including lift distribution, stall characteristics, and induced drag.
Context Anchor
Seen in aerodynamics and lift discussions when comparing different wing shapes and how they affect aircraft performance.
Derivation
From 'plan' (a view from above, as in a floor plan or map) and 'form' (shape). A planform is literally the wing's shape as drawn on a flat plan view.