The Collaborative International Dictionary of English v.0.48:

Tophus \To"phus\, n.; pl. Tophi. [NL.: cf. F. tophus a mineral
   concretion in the joint. See Toph.] [Written also tofus.]
   1. (Med.) One of the mineral concretions about the joints,
      and in other situations, occurring chiefly in gouty
      persons. They consist usually of urate of sodium; when
      occurring in the internal organs they are also composed of
      phosphate of calcium.
      [1913 Webster]

   2. (Min.) Calcareous tufa.
      [1913 Webster]