Bomb - Game Design Wiki

A bomb is a pivotal game event. It is defined by a reward greater than the resources required to generate it. What prevents these major events from un-balancing a game is that all players are aware of the bomb, and often what player actions, collectively, will trigger it. The resulting competition can make benefiting from a bomb harder than might appear from the rules alone.

This is laid out in much more detail in an article by Jonathan Degann.

Examples:

?Vino is a textbook example. The government giveaways occur when all other spaces in a territory are occupied, by any combination of players.
The 'big buildings' in ?Puerto Rico have a huge impact on a players score, and each can only be bought once.
Conflicts in ?Tigris & Euphrates can create large swings in player position.
In Hearts and related card games, ?ShootingTheMoon is a bomb.
In Magic: The Gathering, costly, powerful cards (like Memnarch) are bombs.

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