A geologic formation or stratum that significantly retards fluid movement.
A layer of rock having low permeability that stores groundwater but delays its flow.
A geological formation that prevents significant flow of water (e.g. clay).
aquitard 1. A leaky confining bed that transmits water at a very slow rate to or from an adjacent aquifer. 2. A bed of low permeability adjacent to an aquifer. An aquitard may serve as a storage unit for groundwater, although it does not yield water readily.
Geological formation that may contain groundwater but is not capable of transmitting significant quantities of it under normal hydraulic gradients. May function as confining bed. 
A material of low permeability that greatly slows the movement of ground water.
A rock with limited permeability that allows some water to pass through it, but at a very reduced rate.