Bubble

The bubble is the phase of a tournament just before the money, when the last few players are eliminated before the payouts begin. The term can also be extended to refer to the last few eliminations before any steep rise in the payouts.

Sit-and-gos
Bubble play takes on extra importance because of the contracted nature of sit-and-gos. Because a S&G is essentially only the latter stages of a tournament with a much steeper payout, those stages take on extra importance, beginning with the bubble. For a 10-player S&G, the bubble could be considered to begin with 5 or 6 people.

Multi-table tournaments
One salient difference between sit-and-gos and multi-table tournaments concerns the payout structure. S&Gs only pay three places and usually pay 20% to third, so it's more important to avoid risk that might leave you out of the money. MTTs can pay as much as 20 or 30 times more to first place than to the last paid place, so gambling a percentage or two of the prize pool to finish farther up is worthwhile. [1]

Format variations
Some tournaments mandate hand for hand play whilst on the bubble in a large MTT. This is to prevent short stacks in a MTT from getting into the money simply from engaging in excessively slow play.

Discussion threads

 * A bad way to play on the bubble and The old coin-flip debate (2+2)
 * Fundamental SnG bubble theory question
 * On the Bubble - intereting puzzle

[1]Ed Miller, Getting Started in Hold 'em.