(motor racing) Holding the last qualifying position with qualification still in progress, and thus liable to lose that position.
Smith was on the bubble but lost out when Jones was faster by a tenth of a second.
(sports) Having qualification for an event depend on the upcoming performances of other competitors; hence, at risk of being eliminated, and reversely, being close to qualification; on the verge.
The Knights had been on the bubble for the tournament, but played their way in by winning their last five games.
(idiomatic) Uncertain of success.
The new bond issue now appears to be on the bubble, with support eroding.