Plain English Meaning
A large, pyramid-shaped pointer near the runway that turns with the wind. The pointed end shows the direction you should land — you land toward the point, into the wind.
Full Definition
A wind direction indicator at non-towered airports, shaped like a four-sided pyramid mounted on a pivot, that swings to point into the wind so arriving pilots can identify the favored landing direction.
Context Anchor
Seen on the airport surface at some smaller airports, often near other wind and landing-direction indicators.
Derivation
From the Greek 'tetra' (four) and 'hedra' (seat or face), meaning a shape with four faces. The landing aid is named for its four-sided pyramid form.