A hammock activity (also hammock task) is a schedule or project planning term for a grouping of tasks that "hang" between two end dates it is tied to.
A hammock activity can group tasks which are not related in the hierarchical sense of a Work Breakdown Structure, or are not related in a logical sense of a task dependency where one task must wait for another.
Usages include:
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.
A hammock activity is regarded as a form of Summary activity that is similar to but different from a Level of Effort (LOE) activity. Use of hammock activities is also a way to simplify the difficulties of performing Work Breakdown Structure decomposition to low levels.