Final answer:
Water activity is the ratio of the vapor pressure of a food sample to the vapor pressure of pure water.
Step-by-step explanation:
The ratio of the vapor pressure of a food sample to the vapor pressure of pure water is known as water activity.
Water activity (aw) is a measure of the available moisture in a substance, with pure water having a water activity of 1.0. Microorganisms, such as bacteria and fungi, require specific water activity levels for growth and survival. Decreasing the water content of foods through methods like drying or increasing osmotic pressure can help prevent spoilage.