Plain English Meaning
The moment in a holding pattern when the holding point is straight out to the side of the aircraft, level with the wingtip rather than ahead or behind.
Full Definition
The point in a holding pattern at which the aircraft is directly off the wingtip of the holding fix, ninety degrees to the inbound holding course. In a standard holding pattern, the abeam position marks the start of the outbound leg timing when the holding fix cannot be observed at the wingtip directly.
Context Anchor
Seen in instrument holding procedures, especially when deciding when to start timing the outbound leg.