1. Consisting of or resembling mother-of-pearl 
2. Having a play of lustrous rainbow colours
   "nacreous clouds looking like mother-of-pearl"
   - iridescent, opalescent, opaline, pearlescent

See also: bright, nacre

• ontologist
• ontology
• onus
• onward
• onward motion
• onwards
• onya
• onycha
• Onychium
• Onychogalea
• onycholysis
• Onychomys
• Onychophora
• onychophoran
• onychosis
• onymous
• onyx
• onyx marble
• onyxis
• oocyte
• oodles
• oogenesis
• ooh
• oolite
• oology