Fat-Free Mass (FFM) refers to the total weight of your body minus all of its fat tissue. Your body composition is generally split into two primary parts: fat mass and FFM.
FFM is highly dynamic and serves as the literal engine of your metabolism. It directly impacts how many calories you burn, your appetite, and how your body handles aging. What Comprises Fat-Free Mass?
FFM is often mistakenly used interchangeably with “muscle mass,” but it is actually much more comprehensive. FFM includes everything in your body that is not fat: National Institutes of Health (.gov)