The Collaborative International Dictionary of English v.0.48:

Pensiveness \Pen"sive*ness\, n.
   The state of being pensive; serious thoughtfulness;
   seriousness. --Hooker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pensiveness
    n 1: persistent morbid meditation on a problem [syn:
         pensiveness, brooding]
    2: deep serious thoughtfulness [syn: pensiveness,
       meditativeness, contemplativeness]