Overlay
An overlay occurs when a poker room has to add money to the prize pool in a poker tournament.
Suppose a poker room runs a $100,000 guaranteed prize pool tournament. The buy-in is $100+$9, and only 800 players enter. Normally, the prize pool would be $80,000. Since the poker room guaranteed the prize pool would be at least $100k, it would have to add $20k to the prize pool.
This additional $20k is considered the overlay. All prizes in this tournament are 25% larger than they normally would be had there been no guarantee.
See also: Freeroll.
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