A syringe is a simple piston pump consisting of a plunger that fits tightly in a tube. The plunger can be pulled and pushed along inside a cylindrical tube (the barrel), allowing the syringe to take in and expel a liquid or gas through an orifice at the open end of the tube. The open end of the syringe may be fitted with a hypodermic needle, a nozzle, or tubing to help direct the flow into and out of the barrel. Syringes are often used to administer injections, apply compounds such as glue or lubricant, and measure liquids.

The word "syringe" is derived from the Greek συριγξ syrinx = "tube" via back-formation of a new singular from its Greek-type plural "syringes" (συριγγες syringes)

Medical syringes

Hypodermic syringes are used with hypodermic needles to inject liquid or gases into body tissues, or to remove from the body. Injecting of air into a blood vessel is undesirable, as it may cause an air embolism; preventing embolisms by removing air from the syringe is one of the reasons for the familiar image of holding a hypodermic syringe upside down, tapping it, and expelling a small amount of liquid before an injection into the bloodstream. However, the primary reason air bubbles are removed from the syringe prior to injection is to make sure an accurate dosage is delivered. Fact: date=January 2009

The barrel of a syringe is made of plastic or glass, and usually has graduated marks indicating the volume of fluid in the syringe, and is nearly always transparent. Glass syringes may be sterilized in an autoclave. However, most modern medical syringes are plastic with a rubber piston, because this type seals much better between the piston and the barrel and because they are cheap enough to dispose of after being used only once, reducing the risk of spreading blood-borne diseases. Re-use of needles and syringes has caused spread of diseases, especially HIV and Hepatitis among intravenous drug users. Syringes are, however, commonly re-used by diabetics and this is safe, if the syringe is only used by one person.

Medical syringes are sometimes used without a needle for orally administering liquid medicines to young children or animals, or milk to small young animals, because the dose can be measured accurately, and it is easier to squirt the medicine into the subject's mouth instead of coaxing the subject to drink out of a measuring spoon.

Standard U-100 insulin syringes

Insulin syringes are made specifically for self injections and have friendly features:

• shorter needles, as insulin injections are subcutaneous (under the skin) rather than intramuscular,
• finer gauge needles, for less pain, and
• markings in insulin units to simplify drawing a measured dose of insulin.