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The weak-field dielectric dispersion ͑100 Hz-1.8 GHz͒ studies both of pure and Fe 2ϩ /Fe 3 modified Cd 2 Nb 2 O 7 ceramics over the temperature range of 90-380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that In the latter, at T C ϭ196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd 2 Nb 2 O 7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior ͑like PMN͒. Near the characteristic freezing temperature of the zero-field-cooled state (T f ϭ183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below T f the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd 2 Nb 2 O 7 dominating far below T C ͑around 150 K͒ and that of Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 between 90 and 380 K are typical of glass-forming systems at temperature far above T glass . The relaxational process is characterized by ͑i͒ a significant frequency dependence of the peak permittivity position, ͑ii͒ non-Arrhenius behavior, and ͑iii͒ increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd 2 Nb 2 O 7 pyrochlore and Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation ͑1 MHz-1.8 GHz͒ observed both in Cd 2 Nb 2 O 7 and its isostructural analog Cd 2 Ta 2 O 7 at RT and higher ͑a centrosymmetric phase͒ is attributed to fluctuations in polarization of the dynamically reoriented O͑7th͒-Cd-O͑7th͒ dipoles due to dynamical off-center location of Cd ions.