Plain English Meaning
A cockpit instrument that shows how fast the aircraft is moving through the air around it.
Full Definition
A pitot-static flight instrument that displays the aircraft's speed through the surrounding air, typically in knots. It works by comparing ram air pressure (collected by the pitot tube) with static air pressure (collected from a static port), and mechanically converting the difference into a speed reading on a dial.
Context Anchor
Seen on the instrument panel. Before engine start, the pilot checks that the ASI reads zero; in flight, the pilot uses it during , , , and .