"Silt" is composed of particles in siliciclastic sediment that range in size from 0.0039 millimeters (very fine silt) to 0.0625 millimeters (coarse silt), according to the Udden-Wentworth scale. In field geology, a mudrock has silt in it if it feels smooth to the fingers but feels gritty on the tongue. Silt and clay are collectively classified as mud.
Some of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.