What term refers to administering a substance directly into a vein?

The term that refers to administering a substance directly into a vein is "intravenously." This method involves the direct infusion of fluids, medications, or nutrients into the bloodstream, which allows for fast absorption and immediate effect throughout the body. Intravenous administration is commonly used in medical settings for delivering medications, hydration therapies, or blood products.

Other options describe different routes of administration. For instance, administering "orally" refers to taking substances by mouth, where they are absorbed through the digestive system, which is much slower compared to intravenous routes. "Intramuscularly" involves injecting substances into a muscle, where they are also absorbed at a slower rate than intravenous delivery. Lastly, "subcutaneously" means delivering substances into the layer of fat under the skin, which also provides slower absorption compared to directly injecting into a vein. Understanding these different administration routes is crucial for medical professionals to determine the most effective way to deliver treatments based on a patient’s condition.