Bioavailability means the rate and extent to which the active substance or active moiety is absorbed from a pharmaceutical form and becomes available at the site of action.
In the majority of cases substances are intended to exhibit a systematic therapeutic effect, and a more practical definition can then be given, taking into consideration that the substance in the general circulation is in exchange with the substance at the site of action:
Bioavailability is understood to be the extent and the rate to which a substance or its active moiety is delivered from a pharmaceutical form and becomes available in the general circulation.
It may be useful to distinguish between the absolute bioavailability of a given dosage form as compared with that (100%) following intravenous administration (e.g. oral solution vs. iv.). and the relative bioavailability as compared with another form administered by the same or another non intravenous route (e.g. tablets vs. oral solution).