collapsing

In a project's outline, a mechanism to hide normal tasks beneath their summary tasks, to see just relevant levels of detail. Usually achieved using the Hide Subtasks button. Can also be used to collapse by group and to collapse assignment detail (on task usage and resource usage views). See also expanding.

SOURCE; projectlearning.net

In an outline, hiding the tasks that are subordinate to summary tasks, leaving only the summary tasks visible.

SOURCE: apm.com