The proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect.
This is the dictionary.com definition and it is perfectly fine. From a nutritional perspective, it changes a bit:
The commonly accepted definition of bioavailability is the proportion of the nutrient that is digested, absorbed and metabolized through normal pathways.
Per the Journal of Nutrition
So, why is Bio-Availability important to you?
Easy – naturally occurring minerals are more readily available than those added to water or other beverages. The interesting part about things like reverse osmosis water (where minerals are removed for purity) is that this also removes the flavor. Some waters that want to be seen as “better” add minerals back in – this creates a better flavor profile but does not necessarily improve how well your body can absorb the minerals (you may hear them referred to as electrolytes).
Want to know more? Check out Wikipedia on Bio-Availability: https://en.wikipedia.org/wiki/Bioavailability
Natural mineral waters tend to be higher in minerals than other types of water (sort of obvious really) and they have the added benefit of providing a better overall Bio-Availability than beverages with minerals or electrolytes added. Magnesium and Calcium are two minerals that not only help your body perform better – but can also help your body absorb other nutrients (especially Calcium). Links below to two research papers outlining the benefits: