The Collaborative International Dictionary of English v.0.48:

groundsheet \ground"sheet`\ (ground"sh[=e]t`), n.
   a piece of waterproofed cloth spread on the ground (as under
   a tent) to protect the occupants from moisture.

   Syn: ground cloth.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

groundsheet
    n 1: a waterproofed piece of cloth spread on the ground (as
         under a tent) to protect from moisture [syn: groundsheet,
         ground cloth]