Asceticism is the practice of self-denial (i.e., control of one's passions and base impulses) for the sake of the Kingdom. The practice of asceticism - called ascesis - is most often associated exclusively with monasticism, although all the faithful are exhorted to practice lesser forms of ascesis through the Church's regimen of prayer, fasting, and repentance.
The word "ascetic" comes from the Greek root ἀσκητικός, which is turn is from the verb ἀσκέω, meaning "I train." The Apostle Paul likens the Christian life of prayer and repentance to training for various sporting events (1 Cor. 9:24-27; 2 Tim. 4:7). As such, the methods of ascesis should not be used as ends to themselves, but as means to the end of salvation, the "prize" which the Apostle mentions in First Corinthians.
Some forms of ascesis take a much more austere - even seemingly unhealthy - appearance than others, for instance Stylitism, in which the ascetic stands on a high pillar or tree for a prolonged period of time.