The Collaborative International Dictionary of English v.0.48:

Postmeridian \Post`me*rid"i*an\, a. [L. postmeridianus; post
   after + meridianus. See Meridian.]
   1. Coming after the sun has passed the meridian; being in, or
      belonging to, the afternoon. (Abbrev. P. M.)
      [1913 Webster]

   2. Fig., belonging to the after portion of life; late. [R.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

postmeridian
    adj 1: after noon [ant: antemeridian]