Source : Webster's Revised Unabridged Dictionary (1913)

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.)

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

Source : WordNet®

postmeridian
     adj : after noon [ant: {antemeridian}]