Countersign is a term used to refer to a signature or WORD that is used to AUTHENTICATE or verify something. It is often used in military and diplomatic contexts, where it is used to confirm the identity of someone or to verify a document. In this context, the crossword solutions for the term countersign are AUTHENTICATE (12 letters), PASSWORD (8 letters), WORD (4 letters), UNDERWRITE (10 letters), ENDORSE (7 letters), and WITNESS (7 letters). AUTHENTICATE means to prove the validity of something, while PASSWORD is a secret WORD or phrase used to gain access to a system. WORD is a single unit of language, while UNDERWRITE means to sign a document as a WITNESS or guarantor. ENDORSE means to approve or support something, while WITNESS means to testify to the truth of something.