Sticky bonus
A bonus that can be played with but never withdrawn; it is removed from your balance when you cash out.
A sticky bonus (sometimes called a non-cashable or phantom bonus) stays in your balance for play, but the bonus amount itself is deducted when you withdraw. Only winnings above it can be cashed out, usually after meeting the wagering requirement.
The opposite is a non-sticky (or "parachute") bonus. Here your real money is used first and the bonus only kicks in once your cash is gone. If you win before touching the bonus, you can usually forfeit the bonus and withdraw your cash and winnings without wagering.
Why it matters
Non-sticky bonuses are generally better for players because they give you a free choice: walk away with real-money wins, or continue with the bonus. Sticky bonuses lock your deposit into the wagering requirement from the start.
Worked example
You deposit $100 and receive a $100 sticky bonus. After meeting wagering, your balance is $260. When you withdraw, the $100 bonus is removed and you receive $160.
With a non-sticky bonus, if your $100 deposit grew to $260 before the bonus was used, you could cancel the bonus and withdraw $260.