Plain English Meaning
The line across the middle of the attitude indicator that stands in for the real horizon outside. The little airplane symbol sits above it, below it, or tilted against it to show whether the nose is up, down, or banked.
Full Definition
The horizontal reference line on the attitude indicator that represents the natural horizon. The miniature airplane symbol on the instrument is positioned relative to this bar to show the airplane's pitch and bank attitude in flight.
Context Anchor
Seen on the attitude indicator during attitude control, especially when flying by reference to instruments instead of the outside view.
Derivation
“Horizon” comes from an old Greek word meaning “boundary,” referring to the line where the earth and sky seem to meet. “Bar” means a straight strip or line. Together, “horizon bar” means the straight line on the instrument that stands in for the real horizon.