"H₂O" 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",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en conventions internationales de la chimie",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Formule chimique du monoxyde de dihydrogène."
      ],
      "id": "fr-H₂O-conv-symbol-jsnEzt3M",
      "topics": [
        "chemistry"
      ]
    }
  ],
  "word": "H₂O"
}

{
  "categories": [
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        "Lexique en conventions internationales de la chimie"
      ],
      "glosses": [
        "Formule chimique du monoxyde de dihydrogène."
      ],
      "topics": [
        "chemistry"
      ]
    }
  ],
  "word": "H₂O"
}