Lean body mass
Lean body mass is total body weight minus fat mass: muscle, bone, organs, water. The common equations estimate it from height and weight, and the three most-used ones disagree by several kilograms for the same person.
Example
For an 80 kg, 180 cm man, Boer gives 61.4 kg, James 62.7 kg and Hume 57.8 kg. Nearly 5 kg of spread, which is why showing one number alone is misleading.