A standard Mavic developed with Hutchinson (and possibly Michelin but I can't recall exactly) for a standardization of tubeless bicycle tire and rim specifications. A true UST rim and tire can run tubeless without any sealant because the rim and tire casing are air tight. The bead of the tire is also specially shaped to mate with the rim bead hook for a secure fit.
Many have found (including myself) that running regular non-UST tires (with sealant) on a UST rim often results in "burping"...where the tire bead actually rolls off the rim and the result is a sudden loss of air pressure (and usually a crash to go with it). Some manufacturers are now making "Tubeless Ready" tires that have the reinforced bead designed to work with UST rims, but require sealant as the tire casing is not sealed. These are not by definition UST, but work well with UST rims and do not have the "burping" issues.