Syn or SYN may refer to:

• The Syn, a band formerly including bassist Chris Squire of Yes
• SYN (band), formed by Japanese musician Halca
• Synyster Gates, lead guitarist of the band Avenged Sevenfold

In Norse mythology, Syn (Old Norse "refusal") is a goddess associated with defensive refusal. Syn is attested in the Prose Edda, written in the 13th century by Snorri Sturluson; and in kennings employed in skaldic poetry. Scholars have proposed theories about the implications of the goddess.

In scientific nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name, although zoologists use the term somewhat differently. For example, Linnaeus was the first to give a scientific name (under the currently used system of scientific nomenclature) to the Norway spruce, which he called Pinus abies. This name is no longer in use: it is now a synonym of the current scientific name which is Picea abies.