The Collaborative International Dictionary of English v.0.48:

Excogitation \Ex*cog`i*ta"tion\, n. [L. excogitatio: cf. F.
   excogitation.]
   The act of excogitating; a devising in the thoughts;
   invention; contrivance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

excogitation
    n 1: thinking something out with care in order to achieve
         complete understanding of it
    2: the creation of something in the mind [syn: invention,
       innovation, excogitation, conception, design]