Lean mass
Lean mass is everything in the body that is not fat — muscle, organs, bone, and the water held within them.
Because it includes water and organs as well as muscle, a change in lean mass is not the same as a change in muscle, and the two are frequently reported as though they were.
Body-composition instruments do not measure lean mass directly. They estimate it, using assumptions that can shift with hydration, recent eating and recent exercise.
What it is not
Lean mass is not muscle mass. A reading that moves by a kilogram overnight has almost certainly measured a change in body water rather than in tissue.