bil·lionth 
(bĭlyənth)
(bĭlyənth)n.
1. The ordinal number matching the number one billion in a series.
2. One of a billion equal parts.
billionth adj. & adv.
- under wing
- underwire
- underwood
- underwool
- underworld
- under wraps
- underwrite
- underwriter
- undescended testicle
- undeserved
- undeservedly
- undesigning
- undesirability
- undesirable
- undesirably
- undetermined
- undid
- undies
- undifferentiated
- undignified
- undine
- undiplomatic
- undiplomatically
- undirected
- undiscriminating