Mental invention is a term used to describe the process of creating new ideas or concepts through the use of one's imagination. It is often associated with creative thinking and problem solving, as it involves coming up with solutions to difficult problems. The crossword solution for the term mental invention is 'IDEA, ' which is a four-letter word that describes a thought or concept. Mental invention is an important skill to have in many fields, as it can help people come up with innovative solutions to complex problems.