MerriamWebster online states four definitions for the term plan. Duden, the famous German dictionary, offers almost the exact same definitions. We’re interested in the second definition: “a detailed formulation of a program of action.”
1. A drawing or diagram drawn on a plane: such as a top or horizontal view of an object, a large-scale map of a small area.
2. A method for achieving an end, an often customary method of doing something, a detailed formulation of a program of action.
3. An orderly arrangement of parts of an overall design or objective.
4. A detailed program (as for payment or the provision of some service), i.e. pension plan.