The Collaborative International Dictionary of English v.0.48:

paperback \pa"per*back`\ (p[=a]"p[~e]r*b[a^]k`), n.
   A book with paper covers; -- contrasted with hardback or
   hardcover.

   Syn: softback, soft-cover.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

paperback \pa"per*back\ adj.
   having a flexible binding; -- of books. Contrasted to
   hardcover or hardbacked or hardbound.

   Syn: paperbacked, soft-bound, softcover, softback.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

paperback
    adj 1: (of books) having a flexible binding [syn: paperback,
           paperbacked]
    n 1: a book with paper covers [syn: paperback book, paper-
         back book, paperback, softback book, softback,
         soft-cover book, soft-cover]