"null sign" meaning in All languages combined

{
  "forms": [
    {
      "form": "null signs",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "null sign (plural null signs)",
      "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": "Set theory",
          "orig": "en:Set theory",
          "parents": [
            "Mathematics",
            "Formal sciences",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The sign ∅, associated with empty sets."
      ],
      "id": "en-null_sign-en-noun-hMchezf6",
      "links": [
        [
          "set theory",
          "set theory"
        ],
        [
          "sign",
          "sign"
        ],
        [
          "∅",
          "∅"
        ],
        [
          "associated",
          "associated"
        ],
        [
          "empty set",
          "empty set"
        ]
      ],
      "raw_glosses": [
        "(set theory) The sign ∅, associated with empty sets."
      ],
      "topics": [
        "mathematics",
        "sciences",
        "set-theory"
      ]
    }
  ],
  "word": "null sign"
}

{
  "forms": [
    {
      "form": "null signs",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "null sign (plural null signs)",
      "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",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Set theory"
      ],
      "glosses": [
        "The sign ∅, associated with empty sets."
      ],
      "links": [
        [
          "set theory",
          "set theory"
        ],
        [
          "sign",
          "sign"
        ],
        [
          "∅",
          "∅"
        ],
        [
          "associated",
          "associated"
        ],
        [
          "empty set",
          "empty set"
        ]
      ],
      "raw_glosses": [
        "(set theory) The sign ∅, associated with empty sets."
      ],
      "topics": [
        "mathematics",
        "sciences",
        "set-theory"
      ]
    }
  ],
  "word": "null sign"
}