From Longman Dictionary of Contemporary Englishcardboardcard‧board1 /ˈkɑːdbɔːd $ ˈkɑːrdbɔːrd/ noun [uncountable] 
Dstiff thick brown paper, used especially for making boxes a sheet of cardboard
Dstiff thick brown paper, used especially for making boxes a sheet of cardboardExamples from the Corpus
cardboard• She fed the little slips of cardboard one by one into its grinding jaws.• By night it is lined with ragged figures, sleeping on cardboard or scraps of bubble-wrap.cardboardcardboard2 adjective 1 Dmade from cardboard a cardboard box2 [only before noun]REAL/NOT IMAGINARY seeming silly and not real a romantic novel full of cardboard charactersExamples from the Corpus
cardboard• In one street, the pavement is stacked with cardboard boxes of Toshiba television sets.• Most of these romantic novels are full of cardboard characters.• He put the papers into a cardboard folder and tied it with a red ribbon.• The cedar lining that once protected fine cigars from deteriorating is equally efficacious at preserving cardboard rectangles from insect damage.• Then there's the furniture, notably the super-curvy bentwood chairs for Knoll and the cardboard stuff for Vitra.• Frank Gauci steps off the Callisto into the coldest winter ever clutching a cardboard suitcase. 