Willpower, also known as strength or force of will was an individual's ability to make and carry out decisions.
In 2266, the "good" side of Captain James T. Kirk noted in his log that he seemed to have lost his strength/force of will after being duplicated. Spock also observed the decline in a later entry. (TOS: "The Enemy Within")
Willpower could go a long way towards surviving severe injury or illness. Doctor Leonard McCoy described Joe Tormolen as a man who normally didn't give up, yet he lost the will to live anyway. (TOS: "The Naked Time")