Mental giant is a term used to describe someone who is exceptionally intelligent. It can be used to describe someone who has an impressive knowledge of a particular subject, or someone who is a genius in general. The term can also be used to describe someone who is mentally strong and able to think quickly and accurately. The crossword solution for the term mental giant is 'BRAIN', which is a five letter word. BRAIN is the organ responsible for thought, memory, emotion, and other cognitive functions. It is the most complex organ in the human body and is essential for the functioning of the entire body.