An aquitard is a geological formation that has low permeability and transmits water at a much slower rate than an aquifer. While not completely impermeable, aquitards significantly impede the flow of groundwater, often acting as a confining layer between two aquifers. Understanding aquitards is essential in hydrogeology for groundwater modeling and resource management.
Reference: Fetter, C. W. (2001). Applied Hydrogeology. Prentice-Hall.