A saturated but poorly permeable bed, formation or group of formations that does not yield water freely to a borehole or spring. However, an aquitard may store water and may transmit appreciable water to and from adjacent aquifers. Aquitards normally slow down the movement of groundwater and contaminants. Aquitards can also store groundwater and contaminants.
Water affairs Republic of South Africa
http://www.dwa.gov.za/Groundwater/Groundwater_Dictionary/index.html?introduction_aquifer_testing.htm
http://www.stadtentwicklung.berlin.de/umwelt/umweltatlas/edf212_01.htm