The term 'not optional' is used to describe something that is required or NECESSARY. This can be seen in the crossword solutions for the term, which include 'MANDATORY, ' 'NECESSARY, ' 'NEEDED, ' 'ESSENTIAL, ' 'IMPERATIVE, ' and 'OBLIGATORY.' All of these words imply that something is required and cannot be avoided. The term 'not optional' is often used to emphasize the importance of a task or action, as it implies that it must be done in order to achieve a desired outcome.