The property of mass has never received its formal physics definition using the same method used to define energy, momentum, and force. All definable physics properties can be defined using this method: The definition must be an equation that expresses the defined property in terms of a combination of other properties that have been previously introduced to us by direct empirical evidence. The definition must be formed at the time that the property to be defined is first introduced to us by direct empirical evidence.
It is necessary to stress the need for dependence upon direct empirical evidence because theoretical physics allows the use of indirect empirical evidence to serve as equally sufficient compared to direct empirical evidence. That decision is the worst decision ever made by theoretical physics. However, the immediate need is to formally define mass in the same manner that would later be correctly used to define energy, momentum, and force.
Mass is the connection between direct empirical evidence and all of the rest of physics. Mass must be given its formal physics definition to perform its natural function of communicating the guidance provided by direct empirical evidence to all the rest of physics.