This depends on who you ask. Most people use it as bodyweight minus fat weight. However, most clinicians, especially those in the field of athletic study, use the term in reference to muscle weight.
lean mass=total body weight-fat mass-bone mass-internal organ mass.........there is a formula to give you your lean mass based on b.f. and weight measurements.......its in a book i have....i'll post if i can dig it out......in a nutshell its your muscles, tendons, ligaments.....