The term 'MARK' is a four-letter word that can refer to a variety of things. It can be used as a verb, meaning to make a visible impression or trace on a surface, or it can be used as a noun, referring to a symbol, sign, or indication. It can also refer to a person's name, such as MARK Twain, or a MARK in a school gradebook. In the context of the crossword puzzle, it could refer to any of these meanings.