1. Of sound waves with frequency greater than 1000 million hertz 
2. Moving at more than five times the speed of sound

Encyclopedia: Hypersonic

• obsessed
• obsession
• obsessional
• obsessionally
• obsessive
• obsessive-compulsive
• obsessive-compulsive disorder
• obsessive-compulsive personality
• obsessively
• obsessiveness
• obsessivity
• obsidian
• obsolesce
• obsolescence
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• obsolete
• obsoleteness
• obstacle
• obstacle course
• obstacle race
• obstetric
• obstetrical
• obstetrical delivery
• obstetrical toad
• obstetrician