Hole card is a gambling term used in both online and land-based casinos and is most often used during a game of blackjack or poker.
In blackjack, the term hole card refers to a face-down card the dealer gets. If you are playing a standard variation of blackjack, the dealer will receive two cards. Usually, one of them is going to be a face-up card that everyone can see while the other one is known as the hole card. After each player receives their cards, the dealer peeks to see their hole card to check whether they have a natural blackjack.
When it comes to poker, the term is most often used in stud poker. It has a very similar meaning and as it does in blackjack, and is used to refer to face-down cards the players receive at the beginning of a hand.