The Collaborative International Dictionary of English v.0.48:

Pretermission \Pre`ter*mis"sion\, n. [L. praetermissio. See
   Pretermit.]
   1. The act of passing by or omitting; omission. --Milton.
      [1913 Webster]

   2. (Rhet.) See Preterition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pretermission
    n 1: letting pass without notice