The Collaborative International Dictionary of English v.0.48:

Mat \Mat\, n. [AS. matt, meatt, fr. L. matta a mat made of
   rushes.]
   1. A thick flat fabric of sedge, rushes, flags, husks, straw,
      hemp, or similar material, placed on the floor and used
      for wiping and cleaning shoes at the door, for covering
      the floor of a hall or room to protect its surface, and
      for other purposes.
      [1913 Webster +PJC]

   2. Any similar flat object made of fabric or other material,
      such as rubber or plastic, placed flat on a surface for
      various uses, as for covering plant houses, putting
      beneath dishes or lamps on a table, securing rigging from
      friction, and the like.
      [1913 Webster +PJC]

   3. Anything growing thickly, or closely interwoven, so as to
      resemble a mat in form or texture; as, a mat of weeds; a
      mat of hair.
      [1913 Webster]

   4. An ornamental border made of paper, pasterboard, metal,
      etc., put under the glass which covers a framed picture;
      as, the mat of a daguerreotype.
      [1913 Webster]

   Mat grass. (Bot.)
      (a) A low, tufted, European grass (Nardus stricta).
      (b) Same as Matweed.

   Mat rush (Bot.), a kind of rush (Scirpus lacustris) used
      in England for making mats.
      [1913 Webster]