Source : Webster's Revised Unabridged Dictionary (1913)

Obtenebration \Ob*ten`e*bra"tion\, n. [L. obtenebrate to make
   dark.]
   The act of darkening; the state of being darkened; darkness.
   [Obs.]

         In every megrim or vertigo, there is an obtenebration
         joined with a semblance of turning round. --Bacon.