A hieromonk can be either a monk who has been ordained to the priesthood or a priest who has received monastic tonsure. When a married priest's wife dies, it is not uncommon for him to become a monk, since the Church forbids clergy to enter into a second marriage after ordination.
In the church hierarchy, a hieromonk is of higher dignity than a hierodeacon, just as a secular (i.e., married) priest is of higher dignity than a deacon. Within their own ranks, hieromonks are assigned order of precedence according to the date of their ordination. Ranking above Hieromonk is a Hegumen and an Archimandrite.
Forms of address 
The proper title for a hieromonk is, "the Reverend Hieromonk (religious name)". The form of address is, "Hieromonk (name)", "Father Hieromonk (name)", "Father (name)", or, informally, "Father". As with all Orthodox monks, a hieromonk is not addressed by his family name, but only by his religious name. In writing, if it becomes necessary to use his family name—for instance, to distinguish him from another hieromonk with the same religious name—the family name should be placed in parentheses. Example: "Hieromonk John (Smith)". In cultures where a patronymic is customary, monks are never addressed by their patronymic, but only by their religious name.
In other countries and Western Christianity 
In some countries, married clergy are referred to as "white clergy", while monastic clergy are called "black clergy" because monks should always wear black clothing but married clergy in many parts of the world typically wear white (or gray, blue or some other color) cassocks and rasons.