  A search for heavy leptons decaying to a $Z$ boson and an electron or a muon
is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$
TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to
an integrated luminosity of 20.3 fb$^{-1}$. Three high-transverse-momentum
electrons or muons are selected, with two of them required to be consistent
with originating from a $Z$ boson decay. No significant excess above Standard
Model background predictions is observed, and 95% confidence level limits on
the production cross section of high-mass trilepton resonances are derived. The
results are interpreted in the context of vector-like lepton and type-III
seesaw models. For the vector-like lepton model, most heavy lepton mass values
in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass
values in the range 100-468 GeV are excluded.
