Both terms refer to the mixture prepared for fermentation when making homemade rum. However, the term 'mash' is often used to describe a mixture that includes grains, while 'wash' typically refers to a sugar-based mixture. For rum, since it's derived from sugarcane products, the term 'wash' is more commonly used. A popular variant is the 5-gallon rum wash recipe, which is ideal for homebrew enthusiasts.