Subproject

In order to facilitate the completion of a job, those in project management might break the larger job into a series of smaller tasks called subprojects. By dividing the work into smaller units, employees can be assigned to an aspect of the overall project for which they are best suited. The smaller subprojects also prevent employee burnout, since these are not as overwhelming as a large project might seem.
When project management creates a work breakdown structure, the subprojects are likely to be included in it, but they might be referred to as projects and treated as such. This means that even though they are pieces of the overall task to be completed, these subprojects can be considered projects on their own since they are able to be managed and bought from a seller as can any true project.
These smaller units also appear on project schedule network diagrams. There, those in project management will refer to them as subnetworks. It is the same piece of the larger overall assignment for the company which is seen as a subproject in the work breakdown schedule.

This term is defined in the 3rd and the 4th edition of the PMBOK.