noun: (biology, medicine) The extent to which a substance is molecularly available to cells and tissues in a living organism, which depends on factors such as solubility, pH level, and presence of enzymes and cofactors; for example, even if one's ingestion of vitamin B₁₂ is average/typical, one might nonetheless have B₁₂ deficiency if one has poor metabolism of this particular vitamin.
noun: (pharmacology, nutrition) The amount of drug or nutrient which reaches the site of physiological activity after administration.
