A motor-assisted bicycle.    From Wordnet, Princeton University. 

Fuel economy is further optimized through the latest electric motor-assisted steering system which reduces power drain and low resistance tires.    From Wordnik.com.  [Jalopnik] Reference

For example, check out the sleek "City Cruiser" - a motor-assisted electric-powered mountain bike with a cabin for adults, kids and their packages.    From Wordnik.com. Reference

This is Gene Stoltzfus's last essay, completed on Wednesday, March 10, 2010, just before he headed out on his beloved motor-assisted bicycle on the first spring day of the year.    From Wordnik.com.  [i L i n d . n e t] Reference

Cpl. Debbie Postnikoff of the Castlegar RCMP said the man was riding his motor-assisted bicycle north on Columbia Avenue in the southbound curb lane, against the flow of traffic.    From Wordnik.com.  [Kootenay Rockies - News] Reference

It provides you with motor-assisted cycling from 45 minutes to 1.5 hours, depending on how hard you pedal, while an optional 1kg lithium-ion Mangan battery provides assistance for longer.    From Wordnik.com.  [Gizmag Emerging Technology Magazine] Reference