I have recently started playing more tournaments and have come across the term "overlay" used by online poker rooms. Usually it is described in a dollar amount, like "tournament x had a $500 overlay". not sure exactly what this means, but it only seems to come up in tournaments with a guaranteed prize pool.
In poker, an overlay is the gap between a poker tournament's guaranteed prize pool and the actual prize pool generated by entrants.
For example, if a tournament has a guaranteed prize pool of $10,000, a buy in of $100 and 90 players enter, the players will contribute only $9,000 to the prizepool. The rest of the prizepool (in this example $1,000)—made up by the tournament host—would be the overlay for the tournament.