From Matronics

Bingo (or Bingo Fuel): adj. 1. the term used to describe the point where your aircraft must return to an airport before suffering a fuel starvation event. 2. a predetermined fuel figure remaining in litres, gallons, kilograms, pounds, or minutes which will allow safe return to base plus sufficient reserve.

In a formation flight, briefing a bingo fuel level is important for safety. If any airplane in the formation reaches bingo fuel the pilot can simply tell lead ("lead, three is bingo") and lead knows that he must immediately return the formation to base. This is an important briefing point because the wingman airplanes in a formation will generally burn more fuel than lead, so lead can't simply assume that other airplanes have a similar fuel state to his own.