http://en.wikipedia.org/wiki/Bicycle

The first sentence gives you the definition. There is also more information here, including its history, references, and external links (at the bottom).

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What is the definition of a bicycle?

The definition of a bicycle can be found by visiting the provided link to the Wikipedia page on bicycles. Once on the page, you can find the first sentence of the article which provides the definition. Additionally, you can find more detailed information about bicycles, such as their history, references, and external links, if you scroll down the page.