Asynchrony means "not happening at the same time", and asynchronous message passing is a communication model that does not require the sending and receiving to be synchronized, meaning that the sender isn't blocked until the receiver is ready.

A sequential model like the blocking thread can be extended to support concurrency by overloading synchronous calls with asynchronous semantics, meaning that the call site does not create a dependency on the results of the asynchronous computation; instead, a runtime scheduler passes the results to a handler at a later time or asynchronously. A synchronous call with added asynchronous semantics is called an asynchronous call.

Languages that support first-class functions can model asynchronous messaging with continuation passing style by passing a result handler function (continuation) to the asynchronous call.