"a/o" meaning in English

{
  "etymology_text": "A typescript approximation of a percent sign % with the top zero replaced with 'a' for atom.",
  "forms": [
    {
      "form": "o",
      "tags": [
        "canonical"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "o (uncountable)",
      "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": "topical",
          "langcode": "en",
          "name": "Physical chemistry",
          "orig": "en:Physical chemistry",
          "parents": [
            "Chemistry",
            "Physics",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "atomic percent (percent calculated by number of atoms)"
      ],
      "id": "en-a/o-en-noun-eXeqKZYN",
      "links": [
        [
          "physical chemistry",
          "physical chemistry"
        ],
        [
          "percent",
          "percent"
        ]
      ],
      "raw_glosses": [
        "(physical chemistry) atomic percent (percent calculated by number of atoms)"
      ],
      "synonyms": [
        {
          "word": "m/o"
        }
      ],
      "tags": [
        "physical",
        "uncountable"
      ],
      "topics": [
        "chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "a/o"
}

{
  "etymology_text": "A typescript approximation of a percent sign % with the top zero replaced with 'a' for atom.",
  "forms": [
    {
      "form": "o",
      "tags": [
        "canonical"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "o (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English nouns",
        "English uncountable nouns",
        "en:Physical chemistry"
      ],
      "glosses": [
        "atomic percent (percent calculated by number of atoms)"
      ],
      "links": [
        [
          "physical chemistry",
          "physical chemistry"
        ],
        [
          "percent",
          "percent"
        ]
      ],
      "raw_glosses": [
        "(physical chemistry) atomic percent (percent calculated by number of atoms)"
      ],
      "synonyms": [
        {
          "word": "m/o"
        }
      ],
      "tags": [
        "physical",
        "uncountable"
      ],
      "topics": [
        "chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "a/o"
}