Asceticism is a lifestyle characterized by abstinence from worldly pleasures through self-discipline, self-imposed poverty, and simple living, often for the purpose of pursuing spiritual goals. Ascetics may withdraw from the world or continue to be part of their society, but typically adopt a frugal lifestyle, characterized by the renunciation of material possessions and physical pleasures, and also spend time fasting while concentrating on religion, prayer, or meditation. Some individuals have also attempted an ascetic lifestyle to free themselves from addictions to things such as alcohol, smoking, drugs, sex, porn, lavish food, and entertainment.

Asceticism has been historically observed in many religious and philosophical traditions, most notably among Ancient Greek philosophical schools (Epicureanism, Gymnosophism, Stoicism, and Pythagoreanism), Indian religions (Buddhism, Hinduism, Jainism), Abrahamic religions (Christianity, Judaism, Islam), and contemporary practices continue amongst some of their followers. Practitioners abandon sensual pleasures and lead an abstinent lifestyle, in the pursuit of redemption, salvation, or spirituality. Many ascetics believe the action of purifying the body helps to purify the body and soul, and that in doing so, they will obtain a greater connection with the Divine or find inner peace. This may take the form of rituals, the renunciation of wealth and sensual pleasures, or self-mortification in order to pursue spiritual goals.

However, ascetics maintain that self-imposed constraints bring them greater freedom in various areas of their lives, such as increased clarity of thought and the ability to resist potentially destructive temptations. Asceticism is seen in some ancient theologies as a journey towards spiritual transformation, where the simple is sufficient, the bliss is within, the frugal is plenty. Inversely, several ancient religious traditions, such as Zoroastrianism, Ancient Egyptian religion, the Dionysian Mysteries, and vāmācāra (left-handed Hindu Tantrism), abstain from ascetic practices and focus on various types of good deeds in the world and the importance of family life.

The adjective "ascetic" derives from the ancient Greek term áskēsis, which means "training" or "exercise". The original usage did not refer to self-denial, but to the physical training required for athletic events. Its usage later extended to rigorous practices used in many major religious traditions, in varying degrees, to attain redemption and higher spirituality.

Edward Cuthbert Butler classified asceticism into natural and unnatural forms:

Self-discipline and abstinence in some form and degree are parts of religious practice within many religious and spiritual traditions. Ascetic lifestyle is associated particularly with monks, nuns, and fakirs in Abrahamic religions, and bhikkhus, munis, sannyasis, vairagis, goswamis, and yogis in Indian religions.

In the Baháʼí Faith, according to Shoghi Effendi, the maintenance of a high standard of moral conduct is neither to be associated nor confused with any form of extreme asceticism, nor of excessive and bigoted puritanism. The religious standard set by Baháʼu'lláh, founder of the Baháʼí Faith, seeks under no circumstances to deny anyone the legitimate right and privilege to derive the fullest advantage and benefit from the manifold joys, beauties, and pleasures with which the world has been so plentifully enriched by God, who Baháʼís regard as an all-loving creator.

Notable Christian authors of Late Antiquity such as Origen, Jerome, John Chrysostom, and Augustine of Hippo, interpreted meanings of the Christian Bible within a highly asceticized religious environment. Scriptural examples of asceticism could be found in the lives of John the Baptist, Jesus, the twelve apostles, and Paul the Apostle. The Dead Sea Scrolls revealed ascetic practices of the ancient Jewish sect of the Essenes, who took vows of abstinence to prepare for a holy war. An emphasis on an ascetic religious life was evident in both early Christian writings (e.g., the Philokalia) and practices (e.g., Hesychasm). Christian saints, including Paul the Hermit, Simeon Stylites, David of Wales, John of Damascus, Peter Waldo, Tamar of Georgia, and Francis of Assisi practiced asceticism, as well.