  The duality between localized and itinerant nature of magnetism in
$5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report
inelastic neutron scattering measurements, which reveal both local and
itinerant aspects of magnetism in a single crystalline system of
UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum
of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV,
and surprising absence of coherent spin-waves, suggestive of itinerant
magnetism. While the gap closes above the Neel temperature, strong dynamic spin
correlations persist to high temperature. Nevertheless, the size and
temperature dependence of the total magnetic spectral weight can be well
described by local moment with $J=4$. Furthermore, polarized neutron
measurements reveal that the magnetic fluctuations are mostly transverse, with
little or none of the longitudinal component expected for itinerant moments.
These results suggest that a dual description of local and itinerant magnetism
is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$
systems in general.
