What is Button in Poker?

The actual button placed in front of the “dealer” that moves clockwise each hand, indicating where the dealer, or last person to act, would be in games that use a professional dealer. Button (or on-the-button) is also used to describe the dealer position in each hand.

Example: “Gary raised to 7 Under The Gun (UTG) and the action folded to John. John decided to 3-bet AA On-the-button (OTB), and re-raised to 35.”