"nul" meaning in Esperanto

{
  "etymology_templates": [
    {
      "args": {
        "1": "eo",
        "2": "nulo"
      },
      "expansion": "Back-formation from nulo",
      "name": "back-form"
    }
  ],
  "etymology_text": "Back-formation from nulo.",
  "hyphenation": [
    "nul"
  ],
  "lang": "Esperanto",
  "lang_code": "eo",
  "pos": "num",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Esperanto 8OA",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Esperanto BRO4",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Esperanto GCSE0",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Esperanto back-formations",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "name": "Esperanto cardinal numbers",
          "parents": [
            "Cardinal numbers",
            "Numbers",
            "All topics",
            "Terms by semantic function",
            "Fundamental"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Esperanto entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Words approved by the Akademio de Esperanto",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "english": "Water freezes at zero degrees Celsius.",
          "text": "Akvo glaciiĝas je nul celsiaj gradoj.",
          "type": "example"
        }
      ],
      "glosses": [
        "zero"
      ],
      "id": "en-nul-eo-num--RlOc~np",
      "links": [
        [
          "zero",
          "zero"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[nul]"
    }
  ],
  "word": "nul"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "eo",
        "2": "nulo"
      },
      "expansion": "Back-formation from nulo",
      "name": "back-form"
    }
  ],
  "etymology_text": "Back-formation from nulo.",
  "hyphenation": [
    "nul"
  ],
  "lang": "Esperanto",
  "lang_code": "eo",
  "pos": "num",
  "senses": [
    {
      "categories": [
        "Esperanto 8OA",
        "Esperanto BRO4",
        "Esperanto GCSE0",
        "Esperanto back-formations",
        "Esperanto cardinal numbers",
        "Esperanto entries with incorrect language header",
        "Esperanto lemmas",
        "Esperanto numerals",
        "Esperanto terms with IPA pronunciation",
        "Esperanto terms with audio links",
        "Esperanto terms with usage examples",
        "Words approved by the Akademio de Esperanto"
      ],
      "examples": [
        {
          "english": "Water freezes at zero degrees Celsius.",
          "text": "Akvo glaciiĝas je nul celsiaj gradoj.",
          "type": "example"
        }
      ],
      "glosses": [
        "zero"
      ],
      "links": [
        [
          "zero",
          "zero"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[nul]"
    }
  ],
  "word": "nul"
}