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Asynchronous refers to a communication environment where each party receives and processes messages when convenient or possible rather than immediately.

It can be used to describe a human communication environment like e-mail — the sender will send an e-mail, and the recipient will get round to replying to it when convenient; they don't have to reply immediately.

It can also be used to describe a programatic communication environment, for example Ajax is an asynchronous mechanism for requesting small bits of data over HTTP; the result is sent back when the response is complete, not immediately.

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