A poustinia (Russian: пустынь) is a small sparsely furnished cabin or room where one goes to pray and fast alone in the presence of God. The word poustinia has its origin in the Russian word for desert (пустыня). One called to live permanently in a poustinia is called a poustinik (plural: poustiniki).
Originally a Russian Orthodox tradition, the poustinia was introduced to Roman Catholic spirituality by the Catholic social activist Catherine Doherty in her best-selling book Poustinia: Christian Spirituality of the East for Western Man (ISBN 0-87793-084-8), first published in 1975.
Although originating with ancient startsy (wise Russian elders, sg. starets), Catherine's popular book made the concept of poustinia accessible to modern Western men and women. In it, she describes the poustinia as "an entry into the desert, a lonely place, a silent place, where one can lift the two arms of prayer and penance to God in antonement, intercession, reparation for one's sins and those of one's brothers.... To go into the poustinia means to listen to God. It means entering into kenosis — the emptying of oneself." She promotes the poustinia as a place where anyone — in any walk of life — can go for 24 hours of silence, solitude and prayer. Ultimately, however, the poustinik's call is to the desert of one's own heart wherein he dwells with God alone, whether in the workplace or in a solitary locale.
A poustinik is one who has been called by God to live their life in the desert (poustinia), alone with God in the service of humanity through prayer, fasting, and availability to those who might call upon him or her. Those called to life in the poustinia were not uncommon in Russia prior to the suppression of the Church in the early 20th century.
In this Eastern Christian expression of the eremitic vocation the poustinik is not a solitary, but is a part of the local community to which they are called. The poustinik is a servant of God and God's people, in communion with the Church. Historically, one who experienced the call
"...to the poustinia had first, after securing the blessing of their spiritual director, to find a village. He generally did this through pilgrimage and prayer. Once having discovered the village to which he felt God drawing him, the poustinik went to the elders and asked permission to live there as a poustinik. Permission was happily given, as Russians were glad to have a poustinik praying for them. 
The poustinik lives alone praying for his own salvation, the salvation of the world, and particularly for the community that God has blessed him with. Traditionally,
The poustinik was also available to the people. When there were special needs, such as a fire to fight or hay to bring in, the poustinik would help. And whenever anyone had something they wanted to talk about—a question about prayer, a problem, a special joy or sorrow—they could go to the poustinik. 
The poustinik is one who listens, and shares the love to Christ with all whom he encounters, as well as a cup of tea or some food; whatever he has he shares, as God has shared all with him. 
- Poustinia: Encountering God in Silence, Solitude and Prayer ISBN 0-921440-54-5 (The current 3rd edition)
- Poustinia at the Madonna House Apostolate web site
- The Poustinik Madonna House Apostolateru:Пустынь