The Collaborative International Dictionary of English v.0.48:

Cardboard \Card"board`\ (k[aum]rd"b[=o]rd`), n.
   A stiff compact pasteboard of various qualities, for making
   cards, etc., often having a polished surface.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cardboard
    adj 1: without substance; "cardboard caricatures of historical
           figures" [syn: cardboard, unlifelike]
    n 1: a stiff moderately thick paper [syn: cardboard,
         composition board]