Water Activity is used to determine the propensity of a product to grow mold or fungus and is measured using a Hygrometer.
Defined as the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. A product with low water activity (i.e. Instant Coffee) will be less susceptible to biological growth, while a product with high water activity (i.e. Fresh Meat/Fish) will be more susceptible to biological growth. Water activity is not the same as its moisture content, although products with higher moisture content will typically have higher water activity.