Japanese translation: マイルストーン（指標）：mairu-suton(shihyou)
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Explanation: First of all, I agree to all of those answers posted above.
In my sense, this word has a little bit of more meaning, which implies some (like favorable or neccessary) events or points that are understood without being stated. Although in this case, those answers above may be close enough, I hope the word of マイルストーン will be a wasei-eigo in a future. I've heard this word many times in different situations and still been struggling with it.
Good question! Correct me!