Syringe: A medical device that is used to inject fluid into, or withdraw fluid from, the body. A medical syringe consists of a needle attached to a hollow cylinder that is fitted with a sliding plunger. The downward movement of the plunger injects fluid; upward movement withdraws fluid. Medical syringes were once made of metal or glass, and required cleaning and sterilization before they could be used again. Now most syringes used in medicine are plastic and disposable.