  $^{180m}$Ta is a naturally abundant quasi-stable nuclide and the
longest-lived nuclear isomer known to date. It is of interest for, among
others, the search for dark matter, for the development of a gamma laser and
for astrophysics. So far, its excitation energy has not been measured directly
but has been based on an evaluation of available nuclear reaction data. We have
determined the excitation energy of this isomer with high accuracy using the
Penning-trap mass spectrometer JYFLTRAP. The determined mass difference between
the ground and isomeric states of $^{180}$Ta yields an excitation energy of
76.79(55) keV for $^{180m}$Ta. This is the first direct measurement of the
excitation energy and provides a better accuracy than the previous evaluation
value, 75.3(14) keV.
