Hesychasm is the practice of the Jesus Prayer in an effort to attain union with God(theosis) through stillness (Greek hēsychia). It was primarily a movement within Eastern Orthodox Monasticism, although it can be found elsewhere.
A hesychast is someone who practices the Jesus Prayer with stillness and union with God as the goal. Unlike the word mystic, it does not have the connotation of attaining union with God. It remains humble and just claims the practice.
St. John Climacus, an early leader in the Hesychast tradition, wrote, “Let the remembrance of Jesus be present with each breath, and then you will know the value of stillness (hēsychia).”
Hesychasm is the oldest meditation practice in Christianity, dating back to the early desert fathers and mothers. Traces can even be found in the New Testament, like when Paul to “all those who in every place call on the name of our Lord Jesus Christ” (1 Cor. 1:2).
For those who may not know, the short version of the Jesus Prayer is, “Lord Jesus Christ have mercy on me.” And Jesus promised, “If in my name you ask me for anything, I will do it” (John 14:14).