Source : Webster's Revised Unabridged Dictionary (1913)

Histology \His*tol"o*gy\, n. [Gr. "isto`s tissue + -logy.]
   That branch of biological science, which treats of the minute
   (microscopic) structure of animal and vegetable tissues; --
   called also {histiology}.

Source : WordNet®

histology
     n : the branch of biology that studies the microscopic structure
         of animal or plant tissues