Silnylon

A very lightweight fabric consisting of silicone impregnated nylon (usually ripstop). The nylon is often referred to as parachute fabric, though it is not certified for that. Usually around 0.7-1.4 oz per sq. yd.

The silicone impregnation provides for a waterproofing that is much lighter than urethane coatings.

Siltarp is a name also used, though that is a brand name for specific products.