Merriam-Webster.com

as·​cet·​i·​cism | \ ə-ˈse-tə-ˌsi-zəm  \

Definition of asceticism

1: the practice of strict self-denial as a measure of personal and especially spiritual discipline : the condition, practice, or mode of life of an ascetic: rigorous abstention from self-indulgence

Sacrifice, renunciation, asceticism, fasting, returning again to God … : these are inclinations fueled as much by instinct as by religious idealism. — Joyce Carol Oates

Crossing the Atlantic in a small sailboat is an exercise in asceticism, a test of what the human psyche can do without. — Sarah Ballard

2: austerity in appearance, manner, or attitude

His neatly trimmed white beard and runner’s physique suggest the asceticism of a mathematics professor, while his questioning mind signals a mistrust of surface impressions. — Bruce Schoenfeld

In contrast to the chaste asceticism of earlier modernist office towers, the AT&T Building incorporated traditional symbols of imperial power (Roman vaults and arches), prestige (a Chippendale-inspired silhouette), and old money (a huge golden statue in the lobby, which once stood on top of the old AT&T Building).— Witold Rybczynski

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Etymonline.com

From asceticism:

1640s, from ascetic (adj.) + -ism. Sometimes also ascetism (1830).

From ascetic:

[adjective] 1640s, “practicing rigorous self-denial as a religious exercise,” from Latinized form of Greek asketikos “rigorously self-disciplined, laborious,” from asketes “monk, hermit,” earlier “skilled worker, one who practices an art or trade,” especially “athlete, one in training for the arena,” from askein “to exercise, train,” especially “to train for athletic competition, practice gymnastics, exercise,” perhaps originally “to fashion material, embellish or refine material.”

The Greek word was applied by the stoics to the controlling of the appetites and passions as the path to virtue and was picked up from them by the early Christians. Figurative sense of “unduly strict or austere” also is from 1640s. Related: Ascetical (1610s).

[noun] 1650s, “one rigorous in self-denial,” especially as an act of religious devotion; 1670s, Ascetic, “one of the early Christians who retired to the desert to live solitary lives of meditation, self-denial, and prayer,” from ascetic (adj.).

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Discussion/Explanation

Definition #1 fits best with its use within Christianity.

The earliest Christian monks were a solitary sort referred to as the ascetics because of their choice to shove off the world and spend time in isolation (often in remote locations).

These early individuals did not typically stay there forever. It was from among these ascetics that many of the early church leaders where chosen. This behavior meant they simply could not stay out in the remote wilderness any longer.

Later monastics became something quite different as they were specifically named orders focused around particular ideals–mostly under the Roman Catholic church (though not all).