"horizontal proliferation" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "horizontal proliferation (uncountable)",
      "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"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Nuclear warfare",
          "orig": "en:Nuclear warfare",
          "parents": [
            "War",
            "Weapons",
            "Conflict",
            "Military",
            "Violence",
            "Hunting",
            "Tools",
            "Human behaviour",
            "Society",
            "Human activity",
            "Technology",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "Coordinate term: vertical proliferation"
        }
      ],
      "glosses": [
        "The spread of weapons of mass destruction to states that did not previously have them."
      ],
      "id": "en-horizontal_proliferation-en-noun-JgymWFLA",
      "links": [
        [
          "weapons of mass destruction",
          "weapon of mass destruction"
        ],
        [
          "state",
          "state"
        ]
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "sounds": [
    {
      "rhymes": "-eɪʃən"
    }
  ],
  "word": "horizontal proliferation"
}

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "horizontal proliferation (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "Rhymes:English/eɪʃən",
        "Rhymes:English/eɪʃən/9 syllables",
        "en:Nuclear warfare"
      ],
      "examples": [
        {
          "text": "Coordinate term: vertical proliferation"
        }
      ],
      "glosses": [
        "The spread of weapons of mass destruction to states that did not previously have them."
      ],
      "links": [
        [
          "weapons of mass destruction",
          "weapon of mass destruction"
        ],
        [
          "state",
          "state"
        ]
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "sounds": [
    {
      "rhymes": "-eɪʃən"
    }
  ],
  "word": "horizontal proliferation"
}