milestone UK /ˈmaɪlstəʊn/US /ˈmaɪlˌstoʊn/noun 2verb 2forms 3 noun2 •A stone milepost (or by extension in other materials), one of a series of numbered markers placed along a road at regular intervals, typically at the side of the road or in a median. •An important event in a person's life or career, in the history of a nation, in the life of some project, etc.Death is another milestone on their way. — 1933 verb2 •To place milestones along (a road, etc.). •To plan out a project as a series of major steps. formsmilestoned · milestoning · milestones