  Magnetic excitations of the effective spin $S$=1/2 dimerized magnet
Ba$_2$CoSi$_2$O$_6$Cl$_2$ have been probed directly via inelastic neutron
scattering experiments at temperatures down to 4 K. We observed five types of
excitation at 4.8, 5.8, 6.6, 11.4, and 14.0 meV, which are all dispersionless
within the resolution limits. The scattering intensities of the three low-lying
excitations were found to exhibit different $Q$-dependences. Detailed analysis
has demonstrated that Ba$_2$CoSi$_2$O$_6$Cl$_2$ is a two-dimensional spin dimer
system described only by a single dimer site, where the triplet excitations are
localized owing to the almost perfect frustration of the interdimer exchange
interactions and the undimerized spins, even in small concentration, make an
essential contribution to the excitation spectrum.
