Source : Webster's Revised Unabridged Dictionary (1913)

Lithiasis \Li*thi"a*sis\, n. [NL., fr. Gr. ?, fr. li`qos stone.]
   (Med.)
   The formation of stony concretions or calculi in any part of
   the body, especially in the bladder and urinary passages.
   --Dunglison.

Source : WordNet®

lithiasis
     n : the formation of stones (calculi) in an internal organ