What?
"A logical grouping of role, activities, artifacts, and other process guidance in the description of a process."
The Rational Unified Process Made Easy Per Kroll, Phillipe Krutchen. Addison Wesley

There are 6 technical disciplines:-

Business modelling discipline

Requirements discipline

Analysis and design discipline

Implementation discipline

Test discipline

Deployment discipline

And three supporting disciplines:-

Project management discipline

Configuration and change management discipline

Environment discipline

When?When developing with a sequential or waterfall style the risk remains that one of the sequential steps will cause the process to stall or create barriers stopping others functioning.

For example with configuration management, this may be cast in stone at the beginning of the project. Rigidity and inflexibility mean that the rest of development has to be built around it.

In contrast an iterative development framework such as the RUP will constantly revisit the configuration, through its discipline. Note different phases and iterations will make different calls upon each discipline depending on the appropriate need.

Thus Inception will have a large element of Business Analysis with little testing. This will be reversed in Construction