c1 is an exonym of/for c2, used/given by c3.
An exonym is a name externally given to something or someone, especially a place, ethnic group or language. It is the opposite of an endonym, a self-given or internal name (sezme'e or nerme'e). For instance, "美国" is an exonym for the U.S.A., and "Navajo" is an exonym for the Diné people.