Historical Linguistics and Language Families 5, Exercise 1: terms

Proto-language    the common ancestor of a family, seldom known directly and
                  usually reconstructed. Example: Proto-Indo-European.
Daughter language a language descended from a proto-language; "genetically
                  related" means related by descent. Example: the
                  Indo-European languages.
Branch            a smaller group in a family sharing a more recent common
                  ancestor. Example: Germanic within Indo-European.
Isolate           a language with no proven relatives, a family of one.
                  Example: Basque (about 129 isolates today).

Source: Wikipedia, Language family.