  At low temperatures, Tb$_2$Ti$_2$O$_7$ enters a spin liquid state, despite
expectations of magnetic order and/or a structural distortion. Using neutron
scattering, we have discovered that in this spin liquid state an excited
crystal field level is coupled to a transverse acoustic phonon, forming a
hybrid excitation. Magnetic and phononlike branches with identical dispersion
relations can be identified, and the hybridization vanishes in the paramagnetic
state. We suggest that Tb$_2$Ti$_2$O$_7$ is aptly named a "magnetoelastic spin
liquid" and that the hybridization of the excitations suppresses both magnetic
ordering and the structural distortion. The spin liquid phase of
Tb$_2$Ti$_2$O$_7$ can now be regarded as a Coulomb phase with propagating
bosonic spin excitations.
