  We present a study of the plerionic supernova remnant 0540-69.3 in the LMC in
X-ray, radio, optical, and infrared. We find that the shell of 0540-69.3 is
characterized in the X-ray by thermal nonequilibrium plasma with depleted Mg
and Si abundances and a temperature of kT ~ 0.7 keV. This thermal emission is
superimposed with synchrotron emission in several regions. Based on X-ray
spectra and on morphological considerations in all surveyed wavebands, we
conclude that the shell is expanding into a clumpy and highly inhomogeneous
medium. In one region of the shell we find an overabundance of Ne, suggesting
the presence of ejecta near the edge of the remnant. We also see evidence for
reheating of material via a reverse shock originating from the interaction of
the supernova blast wave with a particularly dense cloud in the surrounding
medium. Finally, we perform the first detailed study of the "halo" region
extending 1.2-2.2 pc from the central pulsar. We detect the presence of thermal
and nonthermal spectral components but do not find evidence for mixing or
ejecta. We conclude that the thermal component is not a counterpart to similar
optical and infrared halos and that it is most likely due to the projection of
shell material along the line of sight.
