During the time of the NT the Roman Empire controlled most of the countries surrounding the Mediterranean Sea including Palestine. Caesar was the family name of a series of Roman Emperors all who in some way were related to Julius Caesar. Those mentioned in the NT conclude Augustus, Tiberius and Claudius (Lk 2:1, 3:1, Act 18:2). Later the term became the title by which succeeding Roman Emperors were called.
See also: Roman Empire.