Low muscle mass at older age has been associated with functional impairments, cognitive decline and mortality. The term sarcopenia, coined in 1988, has been used interchangeably to describe low muscle mass, strength, and function. Without a well defined definition, results of studies using the term sarcopenia cannot be compared. Difficulties in defining sarcopenia parallel the history of defining osteoporosis. To understand critical steps that are needed to reach consensus in defining age-related diseases, we have identified milestones in the history of defining osteoporosis and compared these to sarcopenia. As a result, the main missing steps in the process of defining sarcopenia are: specific treatment options, pharmaceutical interest, and public awareness. Similar to osteoporosis being defined as 'low bone mineral density', the term sarcopenia should be reserved for 'low muscle mass'. Consensus must be reached regarding the diagnostic criteria to quantify muscle mass, correction factors, and reference populations used to define cut-off values of muscle mass.