1. Concave on both sides
   - biconcave

See also: concave

• recreative
• recriminate
• recrimination
• recriminative
• recriminatory
• recross
• recrudesce
• recrudescence
• recrudescent
• recruit
• recruiter
• recruiting-sergeant
• recruitment
• recrystallisation
• recrystallization
• rectal
• rectal artery
• rectal reflex
• rectal vein
• rectangle
• rectangular
• rectangularity
• rectifiable
• rectification
• rectified