Staying awake, usually with the aid of caffeine and cold showers, for over 24 hours. Usually a feat performed by overworked students, but also by physicians and programmers, and other individuals who refuse to sleep until having solved some mystery.
An all-nighter is NOT simply staying up until the sun rises, then sleeping.
Jennifer felt like a zombie after having pulled two all nighters in a row during midterms week. "Really, it's not that bad after the 40th hour or so," she said, "...once you get past the hallucinations and the funny twitching thing your legs do."