Hydraulic organ, also commonly known as water organ, is a type of musical instrument that uses water to produce sound. This type of organ has been used since ancient times and is still used today in modern-day performances. Some synonyms for hydraulic organ include hydraulis, water-powered organ, and hydraulophone. The hydraulis was the first type of hydraulic organ invented and was widely used in ancient Greece. Water-powered organ is a more general term that can be used to describe any type of musical instrument that uses water to generate sound. Hydraulophone is a term used to describe modern-day instruments that are specifically designed to produce sound using water pressure.