1. The physiological accessibility of a given amount of a drug, as distinct from its chemical potency; proportion of the administered dose which is absorbed into the bloodstream: The local veterinarian was conscious of the bioavailability of the medications which were prescribed for the horses.
2. The degree to which a drug is in place throughout the body and is ready for action at the desired receptor sites: After receiving an injection, the doctor advised the patient to rest while she monitored the bioavailability of the medication.
3. The extent to which a nutrient or medication can be used by a person: In order to adjust the medications properly, the doctor monitored the bioavailability of the medication that was prescribed for the patient.

Bioavailability is used to determine whether different brand-name drugs, a generic name as opposed to a brand-name drug, or, in some cases, different batches of the same brand-name drug, will produce the same therapeutic effects.