Bioavailability refers to how well a given material is able to be absorbed and used by the body. Vitamins, minerals, antioxidants and other nutrients all have varying bioavailability in the human body, depending on its source and delivery method.  

This concept has become more and more prevalent in the health and medical communities in recent years because unless your food and supplements are bioavailable, your body is not actually using those nutrients, and unfortunately, you might be wasting your money.