Plain English Meaning
The tendency of a moving object to keep doing what it's doing. When the aircraft speeds up or slows down, the moving parts inside the compass don't react instantly, and that brief delay shows up as a wrong heading.
Full Definition
The resistance of a mass to changes in its motion. When an aircraft accelerates or decelerates, the mass of the magnetic compass float resists that change, lagging behind the aircraft's motion. This lag tilts the float relative to Earth's magnetic field and produces a temporary error in the heading shown on the compass.
Context Anchor
Seen in magnetic compass discussions, especially acceleration error on east or west headings.
Derivation
Inertia comes from the Latin iners, meaning 'inactive' or 'sluggish.' It captures the idea that a mass, left alone, resists being sped up, slowed down, or turned. That sluggishness is exactly what causes the compass float to fall behind the aircraft's motion during .