Bioavailability is a measure of the fraction of the administered dose that reaches the systemic circulation.

Absolute bioavailability is the fraction of the total dose that reaches the systemic circulation. It can have values between zero when the drug is not absorbed at all to 1 when all the administered dose reaches the systemic circulation. The absolute bioavailability can be expressed in terms of percentage (0% - 100%).

The relative bioavailability is the bioavailability of a drug product relative to the bioavailability of another drug product (reference standard preparation). The relative bioavailability can have any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.

The first-order absorption rate constant is the rate constant that determines the rate of drug absorption from the site of administration. Larger absorption rate constant results in faster rate of drug absorption.

The absorption rate constant determined after drug administration is an operative rate constant that accounts for all the necessary steps required for drug absorption into the systemic circulation including disintegration, dissolution and absorption.

The first-order absorption rate constant has units of time^-1.