The Collaborative International Dictionary of English v.0.48:

Mn \Mn\, n. (Chem.)
   The chemical symbol for manganese.
   [PJC]

WordNet (r) 3.0 (2006):

Mn
    n 1: a hard brittle grey polyvalent metallic element that
         resembles iron but is not magnetic; used in making steel;
         occurs in many minerals [syn: manganese, Mn, atomic
         number 25]
    2: a midwestern state [syn: Minnesota, Gopher State, North
       Star State, MN]

The Free On-line Dictionary of Computing (30 December 2018):

mn

    The country code for Mongolia.

   (1999-01-27)