n.1. the application of a drug or other agent in the diagnosis or treatment of a disorder. This may be accomplished enterally (via the digestive tract, as with oral administration) or parenterally (via all other means, including subcutaneous, intramuscular, and intravenous injection; rectal and vaginal suppositories; inhalation; and absorption through skin or mucous membranes). 2. the giving of a test for the purpose of obtaining information.