vicennial /vī-senˈyəl/

1. Lasting, or coming at the end of, twenty years
2. Occurring every twenty years

• silly season
• silly seasons
• Silly String®
• silo
• silo'd
• siloed
• siloing
• silos
• siloxane
• siloxanes
• silphia
• silphium
• silphiums
• silt
• siltation
• siltations
• silted
• siltier
• siltiest
• silting
• silts
• siltstone
• siltstones
• silty
• Silurian