Anything that happens mentally is happening in your mind. This adverb refers to brainy activity. If you’re mentally unstable, you need a shrink.
Just as the adverb physically relates to the body, mentally relates to the mind. A mentally tough person is strong-willed. If someone is mentally unfit for a job, he doesn't have the brains to carry it out. A crossword puzzle can be a mentally challenging task. In the Star Wars movies, Jedi knights can mentally move objects just by willing it to happen. This word covers all the things that happen in your head.