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1. A phrase commonly (and thus sometimes humorously) used as a password, especially to gain entrance to something. In The Arabian Nights, "open sesame" is a magical phrase used to open a robbers' den. A: "What's the password?" B: "Um… open sesame?" If those kids are still blocking the door, tell them the password is "open sesame."
2. By extension, something that helps one to achieve a goal or access an opportunity. In this usage, the phrase is preceded by an article (for instance, "an open sesame"). Good grades in high school will be an open sesame to any college you choose.