What is the act of introducing a substance into the body through a needle or similar device called?

The act of introducing a substance into the body through a needle or similar device is specifically referred to as an injection. This process involves the use of syringes or needles to deliver medications, vaccines, or other substances directly into the body, typically into muscle, blood vessels, or under the skin.

Infusion also implies the introduction of fluids but usually refers to a continuous delivery method, often through an IV, rather than a single entry point like an injection. Ingestion involves taking substances orally, and inhalation refers to breathing substances into the lungs. Therefore, injection is the most accurate term for the described method of administration.