To call a raise after there has been a bet and one or more raises.
In no-limit hold’em, if a player raises before the flop and another calls, the second player can be described as having made a "cold call." The big blind is considered a bet, so the bet and raise followed by a call is a "cold call." Meanwhile, if a player raises and another who is in the big blind calls, that isn’t typically referred to as a "cold call."
Similarly, if there is a raise and a reraise before the flop, then another player just calls the reraise, that is also a "cold call" — in fact, that is more typically where you hear the term used, i.e., to describe someone just calling a preflop three-bet.