extrados
noun
ex·tra·dos ˈek-strə-ˌdäs 
-ˌdō;
ek-ˈstrā-ˌdäs
plural extradoses ˈek-strə-ˌdä-səz or extrados ˈek-strə-ˌdōz
or extrados ˈek-strə-ˌdōz -ˌdäs
: the exterior curve of an arch see arch illustration
Word History
Etymology
French, from Latin extra + French dos back — more at dossier
First Known Use
1772, in the meaning defined above