Bioavailability is defined as

Bioavailability refers to the rate and extent of absorption of a drug from a dosage form as determined by its concentration-time curve in blood or by its excretion in urine It is a measure of the fraction (F ) of an administered dose of a drug that reaches the systemic circulation in the unchanged form. Bioavailability of drug injected i.v. is 100%, but is frequently lower after oral ingestion BA=F=Area under curve oral/ AUC IV Ref-KD Tripathi Pharmacology 7th edition (page no;32)