Plain English Meaning
What the wing looks like when you look straight down at it from above. The outline you'd trace if you drew around the wing on a flat piece of paper.
Full Definition
The shape and outline of an aircraft's wing as viewed from directly above or below. Planform includes the wing's overall shape (rectangular, tapered, elliptical, swept, or delta), its span, its chord, and the relationship between them. Different planforms produce different lift, drag, stall, and handling characteristics.
Context Anchor
Seen in aerodynamics discussions when comparing how different wing shapes affect lift, drag, stall behavior, and handling.
Derivation
From 'plan' (a view from above, as in a floor plan) and 'form' (shape). A planform is literally the shape seen in plan view — looking straight down.