  The phrase "negative squared rest mass" can sometimes be found in papers on
neutrinos and frequently occurs in the tachyonic literature. Consequently, some
authors say that "the rest mass of tachyons is imaginary". Besides, the
statement "photons have zero rest mass" is almost common. In terms of
relativity, however, the state of rest cannot be reasonably defined for luxons
and tachyons, and, therefore, today it does not make sense to speak of rest
mass of such objects. It is shown here that the phrases "negative squared
mass", "imaginary mass", and "photon's zero mass" result from applying
bradyonic dynamical relativistic relations to determine properties of luxons
and tachyons; and that this erroneous procedure results from an unfortunate
interpretation of kinematical relativistic relations. It is also shown that the
use of proper relativistic relations, i.e. luxonic or tachyonic ones, gives a
positive quantity having the squared mass dimension. Thus we obtain a nonzero
real quantity having the mass dimension (called masslike quantity), which is
positive for the photon and may (by intuition - should) be positive for other
luxons and for tachyons.
