I think what Moronium may be referring to is the preference among physicists nowadays to mean rest mass when they speak of mass, and to use other ways to reflect the increase in effective mass arising from relative motion. I have certainly been told more than once, old fogey that I am, that "relativistic mass" is not a concept in widespread use nowadays. That doesn't mean it is an invalid concept, just that the consensus seems to be it can be confusing and so "mass" is taken by default to mean rest mass.

Yes, "preference" seems to be the operational word.

I have also been told not to use relativistic mass as it is an outdated term (among the elites) but I still find it useful, along with things like centrifugal force and cycles per second, which are also looked upon by staring down the noses of the more erudite!

Even the explanations of why relativistic mass is not used, are forced to use it!

From Wiki:

*"The term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object. The invariant mass is another name for the rest mass of single particles. The more general invariant mass (calculated with a more complicated formula) loosely corresponds to the "rest mass" of a "system".*

*Thus, invariant mass is a natural unit of mass used for systems which are being viewed from their center of momentum frame (COM frame), as when any closed system (for example a bottle of hot gas) is weighed, which requires that the measurement be taken in the center of momentum frame where the system has no net momentum.*

*Under such circumstances the invariant mass is equal to the relativistic mass (discussed below), which is the total energy of the system divided by c^2 (the speed of light squared).*

*For other frames, the relativistic mass (of a body or system of bodies) includes a contribution from the "net" kinetic energy of the body (the kinetic energy of the center of mass of the body), and is larger the faster the body moves.*

*Thus, unlike the invariant mass, the relativistic mass depends on the observer's frame of reference. However, for given single frames of reference and for isolated systems, the relativistic mass is also a conserved quantity.*

*Although some authors present relativistic mass as a fundamental concept of the theory, it has been argued that this is wrong as the fundamentals of the theory relate to spaceâ€“time.*** There is disagreement over whether the concept is pedagogically useful**.[2][3][4] The notion of mass as a property of an object from Newtonian mechanics does not bear a precise relationship to the concept in relativity.[5] Oxford lecturer John Roche states that r**elativistic mass is not referenced in nuclear and particle physics, and that about 60% of authors writing about special relativity do not introduce it**.[1]"

So, if you want to be included among the 60% who are elite, you don't use it, but for the rest of us slobs it is still OK, I guess