"open index" meaning in English

{
  "forms": [
    {
      "form": "open indices",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "open indices"
      },
      "expansion": "open index (plural open indices)",
      "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": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Physics",
          "orig": "en:Physics",
          "parents": [
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "In the expression x_μ=η_(μν)x^ν, mu is an open index."
        }
      ],
      "glosses": [
        "index (on a tensor) that isn't summed over"
      ],
      "id": "en-open_index-en-noun-LIXIFn05",
      "links": [
        [
          "physics",
          "physics"
        ],
        [
          "index",
          "index"
        ],
        [
          "sum",
          "sum"
        ]
      ],
      "raw_glosses": [
        "(physics) index (on a tensor) that isn't summed over"
      ],
      "topics": [
        "natural-sciences",
        "physical-sciences",
        "physics"
      ]
    }
  ],
  "word": "open index"
}

{
  "forms": [
    {
      "form": "open indices",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "open indices"
      },
      "expansion": "open index (plural open indices)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Physics"
      ],
      "examples": [
        {
          "text": "In the expression x_μ=η_(μν)x^ν, mu is an open index."
        }
      ],
      "glosses": [
        "index (on a tensor) that isn't summed over"
      ],
      "links": [
        [
          "physics",
          "physics"
        ],
        [
          "index",
          "index"
        ],
        [
          "sum",
          "sum"
        ]
      ],
      "raw_glosses": [
        "(physics) index (on a tensor) that isn't summed over"
      ],
      "topics": [
        "natural-sciences",
        "physical-sciences",
        "physics"
      ]
    }
  ],
  "word": "open index"
}