The Collaborative International Dictionary of English v.0.48:

Gumma \Gum"ma\ (g[u^]m"m[.a]), n.; pl. Gummata. [NL. So called
   from its gummy contents See Gum.] (Med.)
   A kind of soft tumor, usually of syphilitic origin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gumma
    n 1: a small rubbery granuloma that is characteristic of an
         advanced stage of syphilis