Thanatos- death drive; drive to return to a state of calm, or an inorganic or dead state.
Freud defined the life and death drives by defining pleasure and unpleasure first. Unpleasure refers to the increase of stimuli. For example, excessive friction on the skin's surface produces a burning sensation or, the bombardment of visual stimuli amidst rush hour traffic produces anxiety. On the opposite end of the spectrum, pleasure is defined as a decrease in stimuli. For example, a calm environment the body enters after having been subjected to a hectic environment. If pleasure increases as stimuli decrease, the ultimate pleasure would be zero stimulus, or death.
