Sticky bonus
Bonus money you can play with but never cash out; it is subtracted from your balance at withdrawal.
A sticky bonus, also known as a non-cashable or phantom bonus, remains in your balance while you play, but the bonus amount is taken off when you withdraw. Only what you have won on top of it can leave, normally once the wagering requirement is met.
Its opposite is the non-sticky, or "parachute", bonus. Your own cash is played first, and the bonus comes into use only after that cash runs out. Win before you touch the bonus and you can usually give it up and withdraw your deposit and winnings with no wagering.
Which type serves players better
Non-sticky offers generally favour the player because they keep a real choice open: take the cash winnings and leave, or carry on with the bonus. A sticky offer ties your deposit to the wagering requirement from the first bet.
Side-by-side example
Deposit $100, receive a $100 sticky bonus, finish wagering with $260. At withdrawal the $100 bonus is deducted and $160 is paid.
Under a non-sticky offer, if that $100 deposit had reached $260 before the bonus came into play, you could cancel the bonus and withdraw all $260.