Reply

In the context of chess, a reply is a response to an opponent's move.

The most obvious example is that when your opponent puts your King in check, your reply is how you get him back out of check. When your opponent attacks one of your pieces, your reply might be moving it out of the way, protecting it, or interposing a less valuable piece between them. When your opponent captures one of your protected pieces, your reply might be retaliation.