What is the action of filling something with a substance called?

The action of filling something with a substance is best described by the term "infuse." To infuse typically implies introducing a substance into something else in a way that allows it to penetrate or mix thoroughly, often through a process of steeping or soaking. This term is often used in contexts such as cooking, where flavors from herbs might be infused into oils or liquids, or in the medical field, where a substance is infused into a patient's body through an intravenous line.

While the other terms also involve the concept of filling or adding substances, they carry different connotations or specific applications. "Fill" is a general term that refers broadly to the act of making something full, without the specific nuances of mixing or blending that "infuse" conveys. "Inject" is more specific to the act of forcibly introducing a substance into a controlled environment, typically involving a syringe or needle. "Pour" implies a flow of liquid from one container to another, which might not involve the same level of integration or penetration into the original substance as "infuse." Thus, "infuse" accurately captures the essence of the action described in the question.