Your question got cut off at "what is a". In card games, "a" could start many terms. Here are the most likely completions with clear definitions:
| Term | Definition |
|---|---|
| Ace | The card ranked 1 (or high above King). Often dual-rank (A-2-3 or Q-K-A). High value in trick-taking; low/high in Rummy. |
| Ante | A forced bet all players make before cards are dealt (common in Poker). |
| Auction | The bidding phase in Bridge, Bid Whist, or Contract Rummy. |
| Active Player | The player whose turn it currently is. |
| Ace-High | A hand with no pair, ranked by its highest card being an Ace. |
| Add-on / Rebuy | In tournament Poker: buying more chips during a designated break. |
| Abandon / Fold | To surrender the hand (Poker) or fail to follow suit when able (revoke). |
| Abyssinia | Obsolete term for the Ace of Spades (from tax stamp era). |
If you meant a specific phrase like "what is a trick?", "what is a meld?", or "what is a wild card?", please complete the question and I'll give a detailed answer!