Sy is a given name, nickname/hypocorism (often of Seymour) and surname which may refer to:
Sydney ( (listen) SID-nee) is the state capital of New South Wales and the most populous city in Australia and Oceania.
Sylvester Enzio Stallone (; born July 6, 1946) is an American actor, director, screenwriter, and producer.
Systemic lupus erythematosus (SLE), also known simply as lupus, is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body.
Synesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
A synthesizer or synthesiser (often abbreviated to synth) is an electronic musical instrument that generates audio signals that may be converted to sound.