among printers, is the power or weight which presses on the tympan (q.v.), to cause the impression of the letters to be given off and transferred to the sheet. (French, plat, flat.)
In type-writing machines, the platen is the feeding roller on which the paper rests to receive the proper impressions.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Platen from Fact Monster: