Abstract: Discrepancy of definition of dielectric constant between dielectric physics and ferroelectric physics is compared. This discrepancy leads to opposite result in explaining dielectric behavior in ferroelectric ceramics：the ferroelectric physics can properly explain the experimental phenomena of the relationship between polarization and dielectric constant,and the dielectric tenability of ferroelectrics,while dielectric physics can not. The reason is due to the softening of one of transversal optic modes in the phase transition regime,while the dipoles decreases dielectric constant and increases loss. The composition fluctuation model of Smolensky for explaining the relaxation behavior in the paraelectric phase is denied in two aspects： symbol error is found in mathematical derivation,and numerical simulation result with different distribution widths of Gaussian function is different from the observed experimental one.

Key words: dielectric constant, ferroelectric physics, dielectric physics