a program that you can launch from any program without having to exit the running program

In the operating system for the Apple Macintosh computer, a Desk Accessory (DA) was a piece of software, originally written as a device driver, conforming to a particular programming model. The purpose of this model was to permit very small helper-type applications to be run concurrently with any other application on the system. This provided a small degree of multitasking on a system that initially didn't have any multitasking ability at all.