Theoretical considerations concerning the Z^{0} mass

We compare two previously introduced theoretical definitions of the ${\mathit{Z}}^0$ mass and show that one of them, frequently used, is gauge dependent in O(${\mathrm{\alpha }}^3$) in a wide class of gauges and in O(${\mathrm{\alpha }}^2$) in the complementary class. We then discuss a slightly modified version that circumvents these problems, is endowed with desirable theoretical properties, and can be identified with the mass measured at the CERN ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ collider LEP. The simple formulation that emerges is applied to illustrate properties of the physical amplitude, related to gauge invariance, and the role played by the two definitions of mass.