The effects of hydrostatic pressure and biasing dc electric field on the relaxor dielectric response of samples of Pb[(Mg1/3Ta2/3)0.95Zr0.05]O3 with 12%, 15%, and 90% B-site cationic order were investigated. Qualitatively similar decreases in the amplitudes of the real part of the dielectric constant (ε′) at both the peak temperatures (Tm) of the ε′(T,ω) response in the high temperature phase, i.e., above Tm, are observed on increasing the three variables: pressure, biasing field, and B-site order - effects that are interpreted in terms of stiffening of the underlying soft ferroelectric mode of the lattice. Strong deviation of the frequency-independent ε′(T) from the Curie-Weiss law above Tm, attributed to correlations among polar nanodomains, gives way to adherence to this law above the Burns temperature Td. This is the temperature where polar nanodomains first make their presence known. The evolution with decreasing temperature below Td of short-range order in the nanodomains is estimated from the ε′(T) response and shows essentially no dependence on the degree of B-site order. The correlation length for the interaction among the polar nanodomains was also estimated from the dielectric data and found to exhibit strong increase (decrease) with temperature (pressure) as T approaches Tm from above - characteristics of perovskite relaxors. A high temperature dielectric relaxation with an activation energy of - 1.27 eV is observed for the 90% ordered sample, but not for the 12% and 15% ordered samples. This relaxation is attributed to increased oxygen vacancies in the 90% sample that form during the long (64 h) annealing time at 1350 °C to achieve this high level of ordering.