"hm²" meaning in Conventions internationales

{
  "categories": [
    {
      "kind": "other",
      "name": "Symboles en conventions internationales",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Conventions internationales",
      "orig": "conventions internationales",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "related": [
    {
      "word": "m²"
    }
  ],
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en conventions internationales de la métrologie",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Symbole de l’hectomètre carré, unité de mesure de surface du Système international (SI), valant 10⁴ mètres carrés."
      ],
      "id": "fr-hm²-conv-symbol-SOWSyGcd",
      "topics": [
        "metrology"
      ]
    }
  ],
  "synonyms": [
    {
      "sense": "hectare",
      "word": "ha"
    }
  ],
  "word": "hm²"
}

{
  "categories": [
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "related": [
    {
      "word": "m²"
    }
  ],
  "senses": [
    {
      "categories": [
        "Lexique en conventions internationales de la métrologie"
      ],
      "glosses": [
        "Symbole de l’hectomètre carré, unité de mesure de surface du Système international (SI), valant 10⁴ mètres carrés."
      ],
      "topics": [
        "metrology"
      ]
    }
  ],
  "synonyms": [
    {
      "sense": "hectare",
      "word": "ha"
    }
  ],
  "word": "hm²"
}