"vertical proliferation" meaning in English

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      "expansion": "vertical proliferation (uncountable)",
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
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        {
          "kind": "topical",
          "langcode": "en",
          "name": "Nuclear warfare",
          "orig": "en:Nuclear warfare",
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            "Weapons",
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      "examples": [
        {
          "text": "Coordinate term: horizontal proliferation"
        }
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      "glosses": [
        "The increase in the stocks of weapons of mass destruction of states that already had them."
      ],
      "id": "en-vertical_proliferation-en-noun-swjr9M07",
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        [
          "increase",
          "increase"
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        [
          "weapons of mass destruction",
          "weapon of mass destruction"
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          "state",
          "state"
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      "tags": [
        "uncountable"
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  "word": "vertical proliferation"
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      "expansion": "vertical proliferation (uncountable)",
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
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          "text": "Coordinate term: horizontal proliferation"
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        "The increase in the stocks of weapons of mass destruction of states that already had them."
      ],
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          "increase",
          "increase"
        ],
        [
          "weapons of mass destruction",
          "weapon of mass destruction"
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          "state",
          "state"
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  "word": "vertical proliferation"
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