one who baptizes; used in the New Testament only as the title of John, the man known for baptizing people in water.
The word is simply the name for a person who does one particular thing. It is built from the verb βαπτίζω, "to dip" or "to immerse," with an ending that means "the one who does it." So it means "the Baptizer," the person who baptizes.
Every time it appears, it is fastened to one man, John. Baptizing was his signature: he stood at the Jordan River calling people to be washed as a sign of turning back to God, and the crowds knew him by it. The title set him apart from everyone else who shared the common name John. Mark can even swap the title for a plain description, calling him "John who was baptizing," which shows the word carries no more than that: the man marked out by what he did.
Because the activity was tied so closely to him, the word became almost a surname. The first-century Jewish historian Josephus refers to him the same way, as "John who was called the Baptizer," a sign that the title was widely recognized and not only a Christian habit of speech.