›(alsomilepost)an important event in the development or history of something or in someone's life: The opening of a branch in New York represented an important milestone for the company.a milestone in sthThey described the discovery as 'a milestone in medical research'.
›a time when a piece of work must be finished or something must be achieved: In Monday mornings, we would set milestones for the week.If they don't meet the first milestone, you withdraw support.