Definition
A fixed horizontal line on the face of the attitude indicator that represents the natural horizon. The miniature aircraft symbol moves in relation to this bar to show the airplane's pitch and bank attitude relative to the earth's horizon.
Plain English
It is the line on the attitude indicator that stands in for the real horizon outside. By looking at where the little airplane symbol sits compared to this line, the pilot can tell whether the nose is pointed up, down, or level, and whether the wings are tilted.
Context Anchor
Seen on the attitude indicator during the instrument scan, especially when maintaining straight-and-level flight without a clear outside horizon.
Derivation
"Artificial" because it is a man-made stand-in for the real horizon, and "horizon bar" because it is literally a bar across the instrument that represents the horizon line you would normally see outside the aircraft.
Why Pilots Care
It gives the pilot an immediate visual reference for pitch attitude when the natural horizon is not visible.
Intuition Check
Do not read artificial as meaning false or unreliable here. It means the instrument is providing a horizon reference when the real outside horizon may not be usable. The bar is not a handle or control; it is a line you read.
Example Sentence 1
As the pilot rolled into a turn, the miniature aircraft tilted while the artificial horizon bar stayed level, showing a 20-degree bank.
Example Sentence 2
During an instrument takeoff the artificial horizon bar remained steady as the aircraft climbed through the clouds.