permafrost

n [U] subsoil that is permanently frozen, eg in polar regions 永冻土, 永冻层（如在极地的）.

periphrastically, periscope, periscopic, perish, perishable, perishables, perished, perisher, perishing, peristyle, peritonitis, periwinkle, perjure, perjurer, perjury, perk, perkily, perkiness, perky, perm, permanence, permanency, permanent, permanent wave, permanent way, permanently, permanganate, permeability, permeable, permeate, permeation, permissible, permissibly, permission, permissive, permissively, permissiveness, permit, permutation, permute,