Definition
The angle between the longitudinal axis of an aircraft and the direction of the relative wind, measured in the horizontal plane. A nonzero sideslip angle means the aircraft is moving partly sideways through the air rather than straight ahead through its nose.
Plain English
The angle between where the aircraft's nose is pointing and the direction the air is actually hitting it from the side. If the air is coming at the nose dead-on, the sideslip angle is zero. If the air is hitting the side of the aircraft, there is a sideslip angle.
Context Anchor
You may see this term in discussions of slips, crosswind landings, yaw control, and aircraft stability.
Derivation
From 'side' (lateral direction) and 'slip' (to slide sideways). The aircraft is literally slipping sideways through the air rather than tracking straight along its nose.
Why Pilots Care
Controls coordinated flight, affects drag during crosswind corrections, and indicates proper alignment with the runway.
Grounding Statement
Picture the airplane’s nose aimed slightly one way while the airplane’s actual movement through the air is slightly from the side.
Intuition Check
Sideslip angle is not bank angle and not sideways distance. It is the angle between where the aircraft is pointed and where the airflow is coming from.
Example Sentence 1
During the crosswind landing flare, the pilot used rudder to align the nose with the runway, intentionally introducing a sideslip angle to keep the aircraft tracking straight down the centerline.
Example Sentence 2
An increase in sideslip angle during a turn signals the need for coordinated rudder input.