1. One billionth 10^(-9); more generally, very small

Encyclopedia: Nano-

• underemployed
• underemployment
• underestimate
• underestimation
• underevaluation
• underexpose
• underexposure
• underfed
• underfeed
• underfelt
• underfoot
• underframe
• underfur
• undergarment
• undergird
• underglaze
• undergo
• undergrad
• undergraduate
• underground
• underground press
• Underground Railroad
• Underground Railway
• undergrow
• undergrowth