Plain English Meaning
The aircraft's nose is pointing downward compared to where the horizon sits. Left uncorrected, this typically results in the aircraft descending and gaining airspeed.
Full Definition
An aircraft attitude in which the nose is pitched below the horizon, indicating the longitudinal axis is angled downward relative to level flight. On the attitude indicator, this appears as the miniature aircraft symbol sitting below the artificial horizon line.
Context Anchor
Used during pitch-control discussions in straight-and-level flight, especially when a pilot is using instruments instead of outside visual cues.