  We conceptually unify the description of resonances existing at metallic
nanoparticles and optical nanowire antennas. To this end the nanoantenna is
treated as a Fabry-Perot resonator with arbitrary semi-nanoparticles forming
the terminations. We show that the frequencies of the quasi-static dipolar
resonances of these nanoparticles coincide with the frequency where the phase
of the complex reflection coefficient of the fundamental propagating plasmon
polariton mode at the wire termination amounts to $\pi$. The lowest order
Fabry-Perot resonance of the optical wire antenna occurs therefore even for a
negligible wire length. This approach can be used either to easily calculate
resonance frequencies for arbitrarily shaped nanoparticles or for tuning the
resonance of nanoantennas by varying their termination.
