A cold call is a slang term for calling a raise or a re-raise without yet being involved in the hand.
For example, if a player UTG raises, another player in CO re-raises, and you decide to call that raise from the BTN, you are cold-calling a 3-bet in this situation.
The same can be said for single-raised pots. If UTG raises and you call their raise from the BTN, it is also cold calling, but it is usually referred to as a call in these situations.