The term resolution is one of the most important in mass spectrometry because it describes the ability to separate peaks in mass spectra. The term resolving power is often used to describe the ability of a mass spectrometer to resolve adjacent peaks in a mass spectrum and is often used interchangeably with resolution. The separation of peaks for singly charged ions can be expressed as a mass difference Δm and the ratio m/Δm is often given as a quantitative measure of the ability of a mass spectrometer to separate ions. Over the past 50 years, several definitions of mass resolution and mass resolving power have been recommended both by the International Union for Pure and Applied Chemistry and the American Society for Mass Spectrometry that define both resolution or resolving power as m/Δm which has led to confusion about the proper use of these terms. The goal of this work is to investigate the origins and use as well as prior and current definitions of resolution and resolving power and make recommendations for the definition of these terms.