The word "church" is a translation of the Greek word ECCLESIA, which refers to a group of people belonging to the same political party, or in this case following the same "king."
The English word "church" comes from the old English CIRICE (the first C pronounced like a K), a transliteration based upon the Greek word KYRIOS, which means "Lord." It is an interpretation of the word ECCLESIA, meaning "group (or "body") of the Lord."
From what I understand, Protestantism is not a denial or rejection of the exercise of Petrine authority by the Bishop of Rome, but the protestation of how that authority is exercised and/or what the Bishop of Rome teaches. Petrine authority is historical and generally not debated by most. Luther did not deny the need for ecclesial authority or where it came from, merely how that authority was being exercised.
Religious movements that claim to be Christian but do not believe in the historicity of Petrine authority are not Protestant, such as the Latter-Day Saints (Mormons), Seventh-Day Adventists, and Jehovah's Witnesses. A group that is Protestant has some historical connection to both the See of Peter and Apostolic authority, but these groups have no such connection. As such they cannot protest against an authority which was never over them nor can they claim to have removed themselves from historic Christianity.
Orthodox Christianity is not Protestant, however, but it is Apostolic. While it does not deny Petrine authority, it does deny its exclusivity (mainly over politcal grounds). The Orthodox community (as well as the Oriental church bodies of old) are in a semi-communion with the See of Peter.