A bike is a pedal-powered two-wheeled vehicle. If you don't have time to walk to work, you might decide to ride your bike instead.
Bike is shorthand for a bicycle or a motorbike. A bike enthusiast might keep a bike rack on her car and a bike helmet in her trunk at all times. Bike is also a verb, so you could say: "I'll bike over to the movie theater — meet you there in fifteen minutes!" This word, as a nickname for bicycle, first appeared in American English in 1882. It came to mean "motorcycle" about 50 years later.