An Etymological Dictionary of Astronomy and Astrophysics

1. Confidence or trust in a person or thing.
   

   2. Belief that is not based on proof.
      

   3. A system of religious belief (Dictionary.com).

Etymology (EN): M.E. feith fei, fai “faithfulness to a trust or promise; loyalty to a person; honesty, truthfulness,” from Anglo-Fr. and O.Fr. feid, foi “faith, belief, trust, confidence; pledge,” from L. fides “trust, faith, confidence, reliance, credence, belief,” from root of fidere “to trust,“from PIE root *bheidh- “to trust, confide, persuade.”

Etymology (PE): Imân, loan from Ar. al-imân “faith, belief, trust.”