The term 'water activity' (aw) describes the (equilibrium)i amount of water available for hydration of materials. When water interacts with solutes and surfaces, it is unavailable for other hydration interactions. A water activity value of one (1) indicates pure water whereas zero (0) indicates the total absence of 'free' water molecules; addition of solutes always lowering the water activity.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7.