An extra bet of half your wager that pays if the dealer has blackjack.
It is offered only when the dealer's upcard is an ace, and pays 2:1 if the dealer has blackjack. Example: with a 10 bet you add 5, and if it wins you get 10. It's profitable only when 10-value cards make up at least 1/3 of the remaining cards; the guideline is TC +3 or higher.
At the table
When the dealer's upcard is an ace, the dealer asks "insurance?" A player with 10 bet places 5 on the insurance line. If the dealer has blackjack, the 5 wins 10 and the original 10 loses, so the player breaks even.