Lean body mass
Also: LBM, fat-free mass
In plain language
The weight of everything in your body except fat — muscle, bone, organs, and water.
Technical definition
Total body mass minus fat mass, comprising skeletal muscle, bone, organs, and body water; commonly assessed by DEXA or bioimpedance and used to distinguish fat loss from muscle loss.
Preserving lean body mass matters during weight loss, since rapid weight reduction can strip muscle along with fat. Trials of weight-loss and growth-hormone-related compounds often report changes in lean mass separately from total weight.