exostosis
Concise Oxford English Dictionary © 2008 Oxford University Press:
exostosis /ˌɛksɒsˈtəʊsɪs/
▶noun (pl. exostoses /-siːz/) Medicine a benign outgrowth of cartilaginous tissue on a bone.
– origin C16: from Gk, from ex- ‘out’ + osteon ‘bone’.
'exostosis' also found in these Oxford entries:

