"orthodox" simply means conforming to established doctrine, often with an indication on hard line conservatism. You can use orthodox to refer to any especially conservative group in any religion. It is very common to talk about orthodox Jews, Catholics and Muslims. In the context of Judaism it's generally used to means Haredi Judaism, but can sometimes include other conservative groups. The phrase The Orthodox Church does refer to the eastern branch of Christianity, but beyond that there is certain amount of ambiguity in the term.