Plain English Meaning
An airplane with two sets of wings, one above the other, instead of just one.
Full Definition
A fixed-wing aircraft with two main wings stacked one above the other, typically connected by struts and bracing wires. The two-wing configuration produces more lift for a given wingspan and allows a stronger, lighter structure than a single wing of the same era's materials, at the cost of higher drag.
Context Anchor
Seen in aviation history, aircraft recognition, and discussions of early airplane design.
Derivation
From the Latin 'bi-' meaning 'two' and 'planum' meaning 'flat surface' -- the early word for a wing was 'plane,' referring to a flat lifting surface. So a biplane is literally an aircraft with 'two wings.'