Back-flowing is a term used to describe the action of a liquid or gas flowing in the opposite direction to its normal flow. There are several synonyms for back-flowing, including reverse flow, retrograde flow, counterflow, and backwash. These terms can be used interchangeably depending on the context in which they are being used. Reverse flow is commonly used in industries such as plumbing and irrigation systems, while retrograde flow is a scientific term used in the study of fluid dynamics. Counterflow is generally used in industries such as heat exchangers, and backwash is commonly used to describe the cleaning process of water filtration systems.