Why pose this question when everyone knows what mass is? The reason is because no one knows what mass is. The reason that physicists do not know what mass is is because it was adopted as an indefinable property right from the time of its introduction. It remains undefined to this day. 

A physics blast from the past:

The uninformative words primary and secondary have, in modern texts, replaced the informative words indefinable and definable. In order to make the basis for my discussion about temperature clearer for readers in general, I quote from:   

College Physics; Sears, Zemansky; 3rd ed.; 1960; Page 1, Chapter 1:

"1-1 The fundamental indefinables of mechanics. Physics has been called the science of measurement. To quote from Lord Kelvin (1824-1907), "I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of Science, whatever the matter may be."

A definition of a quantity in physics must provide a set of rules for calculating it in terms of other quantities that can be measured.  Thus, when momentum is defined as the product of "mass" and "velocity," the rule for calculating momentum is contained within the  definition, and all that is necessary is to know how to measure mass and velocity. The definition of velocity is given in terms of length and time, but there are no simpler or more fundamental quantities in terms of which length and time may be  expressed. Length and time are two of the indefinables of mechanics. It has been found possible to express all the quantities of mechanics in terms of only three indefinables. The third may be taken to be "mass" or "force" with equal justification. We shall choose mass as the third indefinable of mechanics. 

In geometry, the fundamental indefinable is the "point." The geometer asks his disciple to build any picture of a point in his mind, provided the picture is consistent with what the geometer says about the point. In physics, the situation is not so subtle. Physicists from all over the world have international committees at whose meetings the rules of measurement of the indefinables are adopted. The rule for measuring an indefinable takes the place of a definition. ...

Chapter 15, page 286; 15-1:

To describe the equilibrium states of mechanical systems, as well as to study and predict the motions of rigid bodies and fluids, onlt three fundamental indefinables were needed: length, mass, and time. Every other physical quantity of importance in mechanics could be expressed in terms of these three indefinables., We come now, however, to a series of phenomena, called thermal effects or heat phenomena, which involve aspects that are essentially nonmechanical and which require for their description a fourth fundamental indefinable, the temperature. ... "

No particular interest in this question? Do you understand that when mass becomes defined, physics will change radically. It will affect almost everything. Not knowing what mass is is a risky position to be in. That is the position of today's theoretical physics. 

Ok, it has been brought to my attention in a discussion of another question, that if one refutes Relativity theory, then one must find replacements for Einstein's equations. That is not the case. Here is the case: we will know what the cause of gravity is when mass has been made a defined property. That physicists should understand has to be the case. For any readers who may not know: Mass has always been and remains so today a fundamental indefinable property. Physicists don't know what mass is. Until they do, they will not know what the cause of gravity is. Obviously mass is deeply involved with that cause. Instead of rebuke, rebuke, rebuke, etc. Critics please understand that there is no avoiding the necessity of defining mass and temperature, and, for eliminating the circular definition of electric charge. Until physicists do this, they will not know: What is gravity?: What is Clausius' thermodynamic entropy; What is electric charge? The question posed here is: What is mass? I have critics scolding me for not seeing the obvious truth of General Relativity, when those same critics can't define mass!! General Relativity is a substitute to serve in the place of the lack of knowledge that exists because mass remains an undefined property. 

I have two papers listed here that give the reader a view into the cause of gravity. Neither yet offers a rigorous presentation of the derivation of gravity. However, they are sufficient to demonstrate that my fundamentally unified, empirically lead approach gives guidance to understand the effect we call gravity. The origin of the force of gravity will be described in another message. 

Conference Paper Empirical Origins for Force and Acceleration

Research Calculating the Physical Origin of the Universal Gravitation...

My participation in discussion of questions and answers has revealed unwarranted resistance by Relativists in particular to the need to define mass. They don't see the need. They rest their case on the success of Relativity theory for making accurate predictions. That may sound good to many readers, but, successful predictions are not sufficient to justify the correctness of a theory. It is the patterns of changes of velocities of objects observed in empirical evidence that make  those successful predictions possible. So long as mathematical equations are formed to correctly model those patterns, there will be successful predictions. The equations are vulnerable to the inclusion of incorrect properties or incorrect interpretations of properties even when they successfully mimic the patterns of empirical evidence. This should be openly debated. What is temperature, another undefined property, should be debatable. But first and of the greatest importance is that: What is mass? should be freely debated. The choice of interpretation for mass guides the theory that follows. An undefined mass releases theorists from the requirement to directly connect their theoretical choices to empirical evidence. The release occurs because mass is an undefined property having its own its own lack of direct connection to its empirical evidence and the meaning of mass that is revealed by that evidence.  In closing: an undefined mass cannot keep empirically unsupportable theories in check. The imagined meaning of mass cannot even be kept in check. An undefined mass can be manipulated to serve a theory's needs. A defined mass doesn't serve a theory. A defined mass communicates the needs of empirical evidence. No knowing what mass is is a condition that is intolerable for learning the nature of the universe. Remember that all other properties of mechanics are definable in terms of mass, length and clock-time. Mass must be defined correctly because the definitions of all other properties of mechanics are dependent upon it.  

The equation f/m=a shows that empirical evidence is revealing that the ratio of the defined units of force to the defined units of mass must reduce to the units of acceleration. The units of acceleration are an end point and a beginning for physics. They are an end point because they are naturally indefinable. they are a beginning for physics because they are the units of empirical evidence. Empirical evidence is observed to occur only in this three dimensional space where we live. It takes the form of patterns of changes of velocities with respect to time of objects. The 'a' in f=ma is that empirical evidence. It is letting us know that there are at least two other properties besides space and time. Those two other properties are the properties of action. They are cause one and cause two. Cause one is force. Cause two is mass. Force causes an object to change its velocity. Mass causes changes of velocities to vary from object to object. Force is not explicable. No one knows what force is. We only know about effects. Empirical evidence is always about effects. Effects are always learned from patterns in changes of velocities of objects. No one knows what force is, but, f=ma lets us know that we can substitute ma for force. The 'a' we get as empirical evidence. The 'm' is the key to getting us passed our ignorance of force. if we could explain 'm', then, 'ma' would tell us how to represent force in physics equations. In other words, the units of force must be defined in terms of the units of mass and acceleration. We know the units of acceleration, and, we receive guidance from acceleration about how we can proceed to define mass and its units. The choice of units for mass that works are the units of inverse acceleration.

Empirical evidence guides us to know how to define mass. The units of mass are inverse acceleration. This means that force has units of acceleration divided by units of acceleration. Force has units that cancel out. This change in force brings just as great a change to physics equations. Here is an example of how to make use of a unit free force: 

Research Calculating the Physical Origin of the Universal Gravitation...