Plain English Meaning
A line drawn on the controller's radar screen that shows where airplanes flying downwind should be in order to line up properly for landing.
Full Definition
A reference line displayed on an ARTS III or DBRITE radar scope that shows controllers the correct downwind track aircraft should follow when being sequenced into the traffic pattern for a given runway.
Context Anchor
Seen in diagrams of ARTS III or DBRITE radar displays used by air traffic controllers near airports.
Derivation
Downwind' refers to the leg of the traffic pattern flown parallel to the runway in the opposite direction of landing. 'Alignment guide' simply means a visual line used to keep things lined up. Together, it is a line on the scope that helps the controller keep downwind properly aligned.