Servant of God
Servant of God is the title given to a person of the Roman Catholic Church upon whom a pope has opened a cause of sainthood. It is the first step in the process leading to being declared Venerable upon a decree of heroicity or martyrdom and then honored in beatification with the title of Blessed after the confirmation of miracles attributed to the honoree. The final step is canonization where the honoree would receive the title of Saint.
This should not be confused with Servus Servorum Dei (Servant of the Servants of God), one of the titles of the Pope.
In general Christian usage (and in some other religious traditions -- the Arabic name Abdullah عبد الله literally means "servant of God"), the phrase "servant of God" is used as a description of a person believed to be pious in his or her faith tradition. Though it is commonly only formally used for those for whom a process of canonization has started, in fact it is proper to use it of any Catholic -- hence, it is not yet a declaration by the Church at this stage in the process of any actual finding of fact about the person.