A measure of a creature's ability to suffer wounds, and to a lesser extent its current state of health. Generally a creature loses hit points every time it suffers an injury, but regains them both over time and with the intervention of magic, potions, and scrolls. A game may also have a non-magical healing skill to restore hit points.
When entity loses all of its hit points it is dead. Roguelikes differ as to whether having zero of those means death or not. Angband is an example of the latter approach. Steamband goes even a bit further. No hit points means that all damage will start to drain "wound points". Basically, it is a two-tiered system of hit points, designed to reduce the number of instadeath possibilities.
Hit points were borrowed from tabletop role-playing games and specifically Dungeons and Dragons. It should be pointed out that in at least some D&D rule sets one could actually be into negative hit points without actually being dead. However, alone like roguelike hero, such an individual would surely die, so the practical difference is essentially nil.