The Shields parameter, also called the Shields criterion or Shields number, is a nondimensional number used to calculate the initiation of motion of sediment in a fluid flow. It is a nondimensionalization of a shear stress, and is typically denoted  or . This parameter has been developed by Albert F. Shields, and is called later Shields parameter.  The Shields parameter is the main parameter of the Shields formula. It is given by:
where:
*  is a dimensional shear stress;
*  is the density of the sediment;
*  is the density of the fluid;
*  is acceleration due to gravity;
* is a characteristic particle diameter of the sediment.