An intended sequence set down before acting. A plan's worth is in having thought, not in being followed.
Latin planum (a flat surface), from planus (flat, level), via French plan (a ground-plan, map); a plan is imagined as a flat drawing laid out before the building of the thing it describes.
An intended sequence set down before acting on it. A plan's worth lies in the thinking behind it, not in whether it is ultimately followed.