The true meaning of insanity?

Recently, people have adopted this belief that being insane means being "funny" or "random". But we ALL know that's not what it means. I want to know the REAL meaning of insanity. I don't things like "Apple banana Cheesecake, OMG i am like SO crazy! :D" I want to know how the meaning of REAL insanity. I want to know how a real Maniac perceives reality, How they see the world, how reality looks through their point of view. I want to know!