priest: a person consecrated to serve at a sanctuary, offering the sacrifices and mediating between God (or a god) and the worshipers
A priest is defined by what he does at the altar: he offers the sacrifices, tends the holy place, and stands between the worshipers and their God. In the New Testament the word covers everyone who fills that role, so its meaning stays constant while the person it names changes.
Most often it is an ordinary Israelite priest, a descendant of Aaron who serves at the temple, presents the offerings, and rules on what is clean or unclean. The Gospels send the healed to show themselves to a priest, and the Letter to the Hebrews keeps returning to the daily work of these priests to make its argument.
Two uses stretch the word in fresh directions. Hebrews calls Jesus a priest, and not of the ordinary line: he is a priest forever after the pattern of Melchizedek, the ancient priest of God Most High, so his priesthood never passes to a successor. Revelation then calls the whole company of the redeemed priests to God, a people who now have direct access once reserved for the few. The word can even name a priest of a foreign god, as when the priest of Zeus tries to sacrifice to Paul and Barnabas.