The “celebrant” of a Mass is the priest (or bishop, since bishops are also priests) who is the primary “guy in charge” of saying that Mass. There may be other priests present, but there is only one principal celebrant. And, of course, “celebrant” is just the word meaning “a person who celebrates.” The customary English usage, derived from Latin, refers to the performance of a Mass as a celebration (“Mass will be celebrated tomorrow at …”).
The word is not “what” but “who.” The “celebrant” is the main priest or bishop who performs (celebrates) that particular Mass. Other priests may concelebrate (assist), but he’s the main one behind the altar who consecrates the elements. .
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