"one in a million" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "one in a million",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "1686, Gideon Harvey, The Conclave of Physicians, in Two Parts: Detecting Their Intrigues, Frauds, and Plots..., London: for James Partridge, page 43",
          "text": "[…]most commonly break outwardly, and not one in a Million inwardly.",
          "type": "quotation"
        },
        {
          "ref": "1980 June, Dees, Sam (lyrics and music), “One in a Million You”, in One in a Million You, performed by Larry Graham, Warner Bros.",
          "text": "A one in a million, chance of a lifetime ... one in a million you.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Something extremely rare or unlikely."
      ],
      "id": "en-one_in_a_million-en-noun-OMPiAH0g",
      "links": [
        [
          "rare",
          "rare"
        ],
        [
          "unlikely",
          "unlikely"
        ]
      ],
      "synonyms": [
        {
          "word": "hen's tooth"
        },
        {
          "word": "long shot"
        },
        {
          "word": "rarity"
        }
      ],
      "translations": [
        {
          "code": "fi",
          "lang": "Finnish",
          "sense": "Translations",
          "word": "yksi miljoonasta"
        },
        {
          "code": "fr",
          "lang": "French",
          "sense": "Translations",
          "tags": [
            "feminine"
          ],
          "word": "perle rare"
        }
      ]
    }
  ],
  "word": "one in a million"
}

{
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "one in a million",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English nouns with unknown or uncertain plurals",
        "English terms with quotations",
        "Translation table header lacks gloss"
      ],
      "examples": [
        {
          "ref": "1686, Gideon Harvey, The Conclave of Physicians, in Two Parts: Detecting Their Intrigues, Frauds, and Plots..., London: for James Partridge, page 43",
          "text": "[…]most commonly break outwardly, and not one in a Million inwardly.",
          "type": "quotation"
        },
        {
          "ref": "1980 June, Dees, Sam (lyrics and music), “One in a Million You”, in One in a Million You, performed by Larry Graham, Warner Bros.",
          "text": "A one in a million, chance of a lifetime ... one in a million you.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Something extremely rare or unlikely."
      ],
      "links": [
        [
          "rare",
          "rare"
        ],
        [
          "unlikely",
          "unlikely"
        ]
      ]
    }
  ],
  "synonyms": [
    {
      "word": "hen's tooth"
    },
    {
      "word": "long shot"
    },
    {
      "word": "rarity"
    }
  ],
  "translations": [
    {
      "code": "fi",
      "lang": "Finnish",
      "sense": "Translations",
      "word": "yksi miljoonasta"
    },
    {
      "code": "fr",
      "lang": "French",
      "sense": "Translations",
      "tags": [
        "feminine"
      ],
      "word": "perle rare"
    }
  ],
  "word": "one in a million"
}