“…The job is, however, not to set priorities. That is easy. Everybody can do it. The reason why so few executives concentrate is the difficulty of setting 'posteriorities'—that is, deciding what tasks not to tackle—and of sticking to the decision.”
Posteriorities
From Peter F. Drucker, The Effective Executive (Harper & Row, 1967), in the section “Priorities and Posteriorities” within the chapter “First Things First” (pp. 109-110). Drucker coins posteriorities as the things one decides not to do. The page sets up the contrast directly: ranking what to do is easy, and almost no one sticks to the no.
On the next page, Drucker explains why. What an executive postpones, he writes, “one actually abandons.” By the time anyone proposes to revive the project, the right moment has passed, and a project done at the wrong time tends to fail. So the value of setting a posteriority is not in killing the project. Postponing already does that. The value is in stopping the team from carrying a dead project on the agenda, which frees attention for the priority that will actually get done.