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Phrase(s): a milestone in someone’s life [and] an important milestone in someone’s life
a very important event or point in one’s life. (From the [former] stone markers at the side of a road showing the distance to or from a place.) • Joan’s wedding was a milestone in her mother’s life. • The birth of a child is a milestone in every parent’s life.
McGrawhill's American Idioms And Phrasal Verbs