  We theoretically discover purely electronic polarization modes in THz
frequency region in dimer Mott insulators $\kappa$-(BEDT-TTF)$_2$X. The unusual
low-frequency modes arise from the coupling between the oscillation of
intradimer electric dipole moments and that of alternating interdimer bond
orders. These collective motions play an important role in the dynamical
dielectric properties of the dimer Mott insulators. Near the phase boundary of
the dimer Mott transition, the ferroelectric ground state is realized by
introducing electron-lattice coupling.
