Particles in siliciclastic sediment that range in size from 0.0625 millimeters (very fine-grained sand) to 2.0 millimeters (very coarse-grained sand), according to the Udden-Wentworth scale. In field geology, to tell the difference between fine-grained sand and coarse silt, the particle in a rock is "sand" if it can be clearly seen with the naked eye.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.