Madness is foolish, impulsive behavior. It would be madness to ride on the back of your friend's car, standing on the bumper.
A screaming crowd waiting to see their favorite rock star creates an atmosphere of madness — and their enthusiasm itself can be described as madness. You could also say that donating all your money to an animal shelter would be madness, or describe the madness of a busy Thanksgiving Day sale. Madness sometimes takes on its original meaning, "mental instability" or "insanity."