Does the mention of a Muslim in the definition of backbiting restrict it to Muslims only, and is it permissible to mention a disbeliever with what he dislikes?

Gheebah (backbiting) is "your mentioning of your brother with what he dislikes," as stated in the noble Hadith (definition). The brotherhood intended here is the brotherhood of Islam, and it is not permissible to backbite a Dhimmi (protected) disbeliever. The word "your brother" in the Hadith follows the غالب (common case), similar to His — Exalted be He — saying: "And your stepdaughters under your guardianship."