According to The Modern Catholic Dictionary, austerity is...

AUSTERITY
The quality of being severe in one's demeanor or way of life. More commonly in the plural, austerities. These are vigorous forms of bodily penance that persons impose on themselves in order to bring the passions under control, expiate one's own or other people's sins, and help the spirit to more effectively respond to the will of God. Austerities are only a means to perfection and not an end in themselves. They are to be undertaken only under competent spiritual direction. (Etym. Latin austerus; from Greek austeros, harsh, severe.)

And it is a part of Mortification...

MORTIFICATION
The practice of Christian asceticism in order to overcome sin and master one's sinful tendencies, and through penance and austerity to strengthen the will in the practice of virtue and grow in the likeness of Christ. Natural mortification is a normal part of self-discipline; supernatural mortification, based on faith, seeks to grow in holiness through merit gained by cooperating with the grace of God. (Etym. Latin mortificatio, a killing, a putting to death.)

http://www.catholicculture.org/culture/library/dictionary/index.cfm?id=34988