2. A person who renounces material comforts and leads a life of austere self-discipline; especially, as an act of religious devotion: There have been ascetics who have devoted their lives to God with prayer, fasting, and consecration or solemn commitment to helping those who were in need of spiritual and physical help.
3. Etymology: used since about 1646, from Greek asketikos, "rigorously self-disciplined", from asketes, "monk, hermit"; from askein, "to exercise, to train"; originally, "to train for athletic competition, to practice gymnastics, to exercise".
The noun meaning "one of the early Christians who retired to the desert to live solitary lives of meditation and prayer" is from 1673.
Ascetic actually goes back to Greek asketes, "an athlete in training". The Greek word for athletic training is askesis, and from it we get ascesis, which means "rigorous self-discipline" or "training".
Very much the same thing is denoted by "asceticism", which was first used in English by Sir Thomas Browne in 1646.