1. Occurring every second year
   - biennial 
2. Occurring or payable twice each year
   - semiannual, biannual, half-yearly

1. Every two years
   - biennially 
2. Twice a year
   - semiannually

See also: periodic, periodical

• overnight bag
• overnight case
• overnighter
• overpass
• overpay
• overpayment
• overplay
• overplus
• overpopulate
• overpopulation
• overpower
• overpowering
• overpoweringly
• overpraise
• overpressure
• overprice
• overpriced
• overprint
• overproduce
• overproduction
• overproof
• overprotect
• overprotection
• overprotective
• overproud