Route of administration refers to the path by which a drug or other substance is taken into the body. It can include oral (by mouth), intravenous (injected into a vein), intramuscular (injected into a muscle), subcutaneous (injected under the skin), topical (applied to the skin), inhalation (breathed into the lungs), and others. The route chosen depends on factors like the properties of the substance, desired onset of action, and patient preference.