An Etymological Dictionary of Astronomy and Astrophysics

1. Free from darkness, obscurity, or cloudiness.
   

2. Transparent; pellucid.
   

3. Without discoloration, defect, or blemish (Dictionary.com). → clear night.

Etymology (EN): M.E. clere, from O.Fr. cler, from L. clarus “clear, bright, distinct.”

Etymology (PE): Runé, from Kurd. (Sorani) rûn “bright, clear,” rûn kirdin “to explain,” variant of rowšan, → bright.