A Hammock activity is a schedule (project management) or project planning term for a grouping of subtasks that "hangs" between two end dates it is tied to[1].
A Hammock activity groups subtasks which are not related in the hierarchical sense of a Work Breakdown Structure.
A Hammock activity groups subtasks which are not related in a logical sense of a dependency (project management) where one subtask must wait for another.
Usages:
The duration of the hammock activity (the size of the hammock) may also be set by the subtasks within it, so that the abstract grouping has a start date of the earliest of any of the subtasks and the finish date is the latest of any of the contents.