The maximum withdrawal (max cashout) is a limit in the bonus terms on how much of a win arising from the bonus is withdrawable. The rest is deleted at the moment of the withdrawal.
Two forms
A fixed amount — "max withdrawal €100". That is the form which makes a large bonus practically worthless: a €500 bonus with a €100 cap is in reality a €100 offer that takes a €500-bonus worth of clearing to reach.
A multiple — "max 5× the bonus amount". That is fairer, because the cap scales with the offer.
Where it appears most often
On no-deposit bonuses and free spins, almost always. The logic makes sense from the casino's side: when your own money is not at stake, the upper end is capped. See the no-deposit bonus list.
How to factor it into the calculation
An offer's real value is the lower of two: the balance expected after wagering, and the max cashout. If the second is smaller, all the rest of the calculation is theoretical. Use the max cashout calculator and the bonus calculator.
The alternative
Rakeback has no max cashout, because it is not a bonus but a payment — a share of your own stakes back, withdrawable immediately. See the list of houses without wagering.
Example: how the cap cuts the win
Suppose free spins produced a 250 USDT win, but the max cashout is 100 USDT. You can withdraw 100 USDT and 150 USDT is deleted. For a multiplier-based cap: a 50 USDT bonus with a cap of 5x the bonus gives a ceiling of 250 USDT. Always ask whether the cap covers only the bonus win or also your own deposit and its winnings, because the wording differs. For the wagering itself see wagering; an alternative without a cap is rakeback.
Why the cap lowers the value more than it seems
The cap cuts off exactly the biggest wins. Mathematically, the mean of a capped outcome is always less than or equal to the uncapped mean, see Wikipedia: Expected value. In practice a high-volatility game generally loses more because of the cap, since its value comes from rare large wins. So the value of an offer cannot be judged by the wagering alone, but also by the cap.
Maximum cashout is not a withdrawal limit
The two are often confused. Max cashout deletes the rest once the bonus is completed. A withdrawal limit (per day, week or month) deletes nothing, it only spreads the payout over time. For the latter see the limits page, the list of casinos without limits and the post limits when HAMPI does not apply.
Where else to look
The same concept at Estonian-licensed casinos is explained by maximum cashout on freespin.ee, and withdrawal restrictions in general by withdrawal limits on kiiredvaljamaksed.ee.