A hole card is a card dealt face down. In standard blackjack it is the dealer’s concealed second card, sitting next to the face-up card everyone can see. In American-style games, when that up card is an ace or a ten-value card, the dealer checks the hole card for blackjack before players act, so a natural is settled at once. European no-hole-card rules deal the second card only after the players have finished.
In stud poker, hole cards are each player’s private face-down cards, as opposed to the up cards on show, and Hold’em players use the term for their two pocket cards. Keeping hole cards hidden is central to game security. Cheating methods such as spooking or using a shiner try to expose them, and live dealer studios set camera angles so they stay hidden until the reveal.