Casual assent is a term used to describe a response that is neither enthusiastic nor negative. It is often used to indicate agreement or acceptance of something without any strong emotion. In this context, the crossword solutions 'YEAH' (4 letters), 'YEP' (3 letters), and 'Yeh' (3 letters) all refer to casual assent. These words are informal expressions of agreement or acceptance, and can be used to indicate that someone is in agreement with a statement or proposal.