Short for "Excited dimer".
Some atomic gases (like argon, or krypton) mixed with fluorine, have a tendency to form excited compounds, like ArF or KrF when excited by an electric discharge. These compounds are very unstable, and decompose readily into its original componants, with emission of a photon. Note that the correct term should usually be "Exciplex laser", as these excited compounds are not actually dimers. Excimer lasers share some common properties :
Emission in the medium UV (126nm to 558nm).
Short, intense pulse.
Rectangular beam profile.