Asynchronous

Asynchronous communication is communication that occurs at irregular intervals without the expectation of a response. E-mail is an example of asynchronous communication. E-mail messages are neither sent or received at predictable intervals, and you may or may not be notified when a message is received. You can also continue to work with other e-mail regardless of whether you receive a response.

In contrast, synchronous communication occurs when a program sends a request and doesn't take further action until it receives a response.