Monasticism (from the Greek word μοναχός—solitary) is the ancient Christian practice of withdrawal from the world in order to dedicate oneself fully and intensely to the life of the Gospel, seeking union with our Lord Jesus Christ. The focus of monasticism is the soul’s purification, illumination, and deification, or theosis. It is the process of perfection in Christ to which every Christian is called: “Be ye perfect as your heavenly Father is perfect” (Matt. 5:48).
Within their monastery enclosure, monastics live a life of spiritual stillness called hesychasm, working in silence and constant prayer throughout the day, keeping vigil at night, and carefully attending to their thoughts and feelings through inner watchfulness and prayer, while participating continually in the Sacraments and the liturgical life of the Church. At the center of this life lived for Christ is the ceaseless repetition—vocally or silently—of the Jesus prayer: “Lord Jesus Christ, have mercy on me.” This prayer, practiced within the framework of perfect obedience to the monastery’s abbot, brings the grace of God into the disciples’ souls. With patience and perseverance, in time and by the great mercy of God, they attain to the acquisition of the Holy Spirit.
Ceaseless prayer of the heart is a commandment of our Lord Jesus Christ Himself: “And He spake a parable unto them to this end, that men ought always to pray, and not to faint” (Luke 18:1). This form of concentrated, unremitting, noetic prayer was first practiced by the Holy Apostles and then handed down to each generation of Christians, monastics and laity alike, as Saint Paul exhorted: “Pray without ceasing” (1 Thess. 5:17). Dwelling in freedom from distractions and living outside the bustle of the modern world, the monastic offers this prayer to God for the salvation of the entire world.
Concerning the apostolic monastic life, Metropolitan Hierotheos of Nafpaktos writes:


For more on monasticism see the article by Metropolitan Isaiah of Denver, the former President of Hellenic College/Holy Cross Greek Orthodox School of Theology.

Also, see the article by Georgios I. Mantzarides, Professor Emeritus of the Theological School Aristotle University of Thessaloniki (abridged text from the book Images of Athos by monk Chariton); Professor Mantzarides taught Christian Ethics and Sociology and is also a member of the Bioethics Committee of the Holy Synod of the Church of Greece.