NOUN (3)

1. work table of a machine tool;

2. the flat plate of a printing press that presses the paper against the type;

3. the roller on a typewriter against which the keys strike;

The Collaborative International Dictionary of English v.0.48:

Platen \Plat"en\, n. [F. platine, fr. plat flat. See Plate,
   and cf. Platin.] (Mach.)
   (a) The part of a printing press which presses the paper
       against the type and by which the impression is made.
   (b) Hence, an analogous part of a typewriter, on which the
       paper rests to receive an impression.
   (c) The movable table of a machine tool, as a planer, on
       which the work is fastened, and presented to the action
       of the tool; -- also called table.
       [1913 Webster]

WordNet (r) 3.0 (2006):

platen
    n 1: work table of a machine tool
    2: the flat plate of a printing press that presses the paper
       against the type
    3: the roller on a typewriter against which the keys strike