In poker, a cold call refers to the act of calling a raise without having already put in a bet or a raise in the current betting round. For example, if a player bets, and another player then raises, a third player who calls that raise without having previously bet or raised in that betting round is making a cold call.
It is called a “cold” call because the player is entering the pot “cold,” without any prior investment. Cold calling is generally considered to be a risky play, as the player is playing a larger pot with a weaker hand than if they had bet or raised themselves.