A monk may be conveniently defined as a member of a community of men, leading a more or less contemplative life apart from the world, under the vows of poverty, chastity, and obedience, according to a rule characteristic of the particular order to which he belongs. The word monk is not itself a term commonly used in the official language of the Church. It is a popular rather than a scientific designation, but it is at the same time very ancient, so much so that its origin cannot be precisely determined.
A monk is a man living in a religious community and makes a final profession of the solemn vows of poverty, chastity, and obedience. A monk may be a priest or a deacon, who has received the Sacrament of Holy Orders, or a religious brother, who is not ordained. Monks live in a monastery, the word from which "monk" is derived. Depending upon the circumstances of the particular order, they may have a very strict contemplative, cloistered lifestyle, like the Order of Cistercians of Strict Observance (commonly known as the Trappists), or a less strictly cloistered lifestyle, like the Benedictines.
Just as an aside, these monasteries are referred to as abbeys when they are independent, self-sufficient, and have a certain number of monks or nuns. The head of the abbey is either the abbot or abbess.