Lean body mass (fat-free mass)
Everything in the body that is not fat, chiefly muscle; protecting it preserves metabolism.
Lean body mass, also called fat-free mass, is body weight minus fat: muscle, bone, organs, and water. Muscle is the part most relevant to weight maintenance because it drives resting metabolic rate. Rapid or unsupported weight loss takes a share of weight from muscle, which lowers metabolism and makes regain easier. Adequate protein (around 1.5 g/kg/day) and resistance training are the two evidence-based ways to protect lean mass during and after weight loss.