"nm3" meaning in Conventions internationales

{
  "categories": [
    {
      "kind": "other",
      "name": "Variantes par contrainte typographique en conventions internationales",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Conventions internationales",
      "orig": "conventions internationales",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "typographic variant",
  "pos_title": "Variante par contrainte typographique",
  "senses": [
    {
      "alt_of": [
        {
          "word": "³"
        }
      ],
      "glosses": [
        "Variante typographique fautive de nm³, très souvent utilisée, le caractère « ³ » n’étant pas disponible au clavier."
      ],
      "id": "fr-nm3-conv-typographic variant-BRA-HtqT",
      "tags": [
        "alt-of"
      ]
    }
  ],
  "tags": [
    "alt-of"
  ],
  "word": "nm3"
}

{
  "categories": [
    "Variantes par contrainte typographique en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "typographic variant",
  "pos_title": "Variante par contrainte typographique",
  "senses": [
    {
      "alt_of": [
        {
          "word": "³"
        }
      ],
      "glosses": [
        "Variante typographique fautive de nm³, très souvent utilisée, le caractère « ³ » n’étant pas disponible au clavier."
      ],
      "tags": [
        "alt-of"
      ]
    }
  ],
  "tags": [
    "alt-of"
  ],
  "word": "nm3"
}