Plain English Meaning
The moment your aircraft is exactly off to the side of the navigation station on the ground, with the station sitting straight out from your wing rather than ahead of you or behind you.
Full Definition
The position of an aircraft when a navigation station (such as a VOR or NDB) is directly off the wingtip, at approximately a 90-degree angle to the aircraft's track. When abeam a station, the bearing to the station is perpendicular to the aircraft's course.
Context Anchor
Used in instrument navigation when judging your position relative to a ground station during or while a course.