To countersign means to add your signature to a document that someone else has already signed, confirming that it's genuine and you approve it. When a principal countersigns a permission slip that a parent signed, she's verifying that she's seen it and agrees to let the student go on the field trip. When a bank manager countersigns a check, he's confirming the transaction is legitimate.

The word suggests an extra layer of verification and responsibility. A single signature might not be enough for important documents: someone in authority needs to countersign to make it official. A doctor might countersign a medical form a nurse prepared. A commanding officer might countersign orders from headquarters before soldiers carry them out.

In military contexts, a countersign (as a noun) also means a secret password given in response to another password. A guard might challenge someone approaching with “Liberty,” and the person must respond with the countersign “Justice” to prove they belong there. This usage comes from the idea that one password counters or matches the other, like two signatures working together.