From 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]