(also written as Aw) The ratio of the vapour pressure of water in a substance relative to the vapour pressure of pure water under standard conditions. Pure water thus has a water activity of 1 (a 1:1 ratio), while a substance that generates half as much vapour pressure will have a water activity of 0.5. This number is a measure of how much water is available to take part in reactions. A food with low water activity will not go mouldy because the water in it can’t be used by microorganisms.