What is asynchronous data communication?

In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream.

Where is asynchronous used?

Asynchronous programming is a better fit for code that must respond to events – for example, any kind of graphical UI. An example of a situation where programmers use async but shouldn't is any code that can focus entirely on data processing and can accept a “stop-the-world” block while waiting for data to download.