For a percise defination of insanity, read the first wall of text, for an inpercise but simple to understand defination, go to the bottom of the post.
I would define insanity in two as a partial disability of perception (perception = the 5 senses and/or how the mind processes info (which orginate from the 5 senses)). Sorry for the fancy words, it's kind of hard to describe with normal words.
By partial disability of perception I don't mean a person is deaf, blind, etc. but rather than he sees things which aren't there. In fact, the 5 senses are ussualy not the cause of this sort of insanity, although they can be. It's ussually the mind's thought process which interpets these senses which is somehow messed up (meaning he interpets something objective in a way which is simply not true to the objective fact). These insanties can range from subtle (that Einstein example) to dangerous for the man with the insanity or his fellow man (random attacks due to the illusions cast by the mind and/or the senses).
A full disablity in thought process and/or senses is not considired insanity. Being blind is not insane, not being able to think is not insanity, because that means one is either dead, or a vegetable (which doesn't happen due to experiences of a person, but rather through failure of the body, making it a disease, or a condition, but not insanity, which always orginates in the person's mind).
The "irreguluar-mind thinking" defination is too ambiguous. Athiests are a minority in the world - and they are thus to be considered insane because their thought process dictates that there is no god?
Excuse my spelling, I haven't written in fancy english, or in grammartically correct english for quite some time.
And if I confused you, let me say that shit in simple english. Insanity is what happens when your mind gets fudged up because of some weird voodoo shit (can't explain in simple englsih) which makes it self destruct on itself. This self destruction makes you unable to tell reality aport from your imgination, and that is insanity.