The word seraph (pl. Seraphim) is a Hebrew word whose occurrence in the Bible is limited to instances in the Old Testament book of Isaiah. This song refers to the angels hailing the Birth of Jesus, whose story is told nowhere in the Bible except in the New Testament gospel of Luke. Various words or phrases in the Bible are traditionally (at least) interpreted as referring to specific categories of angels. In addition to Seraph there is another Hebrew word, Cherub (pl. Cherubim) that has been interpreted as another category of angel. And then there's Archangel, a term sometimes used in the New Testament to refer to specific angels. And groups of angels are sometimes referred to as "Heavenly host" (rendered "Heaven's armies" in some modern English translations). Since "Seraph" occurs only in the Old Testament, it is questionable at best to interpret any angelic being referred to in the New Testament as a Seraph.