A xerophile is an extremophilic organism that can grow and reproduce in conditions with a low availability of water, also known as water activity. Water activity (a_w) is measured as the humidity above a substance relative to the humidity above pure water (Aw = 1.0). Xerophiles are "xerotolerant", meaning tolerant of dry conditions. They often can survive in environments with water activity below 0.8. Typically xerotolerance is used with respect to matric drying, where a substance has a low water concentration. These environments include arid desert soils. The term osmotolerance is typically applied to organisms that can grow in solutions with high solute concentrations (salts, sugars), such as halophiles.