Fatigue

Fatigue determines how much an actor (NPC or the player) can do before tiring himself out. Doing anything basically decreases the amount of Fatigue and especially in the game of Morrowind conciderably effects the outcome of what you do, mainly lowering the success rate of picking locks, casting spells and so on.

Fatigue is not really an issue in Oblivion at all, but in Morrowind it is easily depleted by running or fighting and only regenerates while doing nothing. There are Restore Fatigue enchanted items though, to counter this. In Oblivion you really only lose a good amount of Fatigue when performing a power attack, and your Fatigue incredibly regenerates even when running!