The Collaborative International Dictionary of English v.0.48:

Mattress \Mat"tress\, n. [OF. materas, F. matelas, LL.
   matratium; cf. Sp. & Pg. almadraque, Pr. almatrac; all from
   Ar. ma[.t]rah a place where anything is thrown, what is
   thrown under something, fr. [.t]araha to throw.]
   [1913 Webster]
   1. A large pad stuffed with hair, moss, or other suitable
      soft material, and quilted or otherwise fastened, used as
      or in a bed, to support the human body while lying down.
      [Written also matress.]
      [1913 Webster]

   2. (Hydraulic Engin.) A mass of interwoven brush, poles,
      etc., to protect a bank from being worn away by currents
      or waves.
      [1913 Webster]

   innerspring mattress A variety of mattress[1] having
      springs inside to provide a flexible support; it is
      considered more comfortable than a stuffed mattress.
      [PJC]

WordNet (r) 3.0 (2006):

mattress
    n 1: a large thick pad filled with resilient material and often
         incorporating coiled springs, used as a bed or part of a
         bed