"relative density" meaning in English

{
  "forms": [
    {
      "form": "relative densities",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "relative density (countable and uncountable, plural relative densities)",
      "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",
          "langcode": "en",
          "name": "Physical quantities",
          "orig": "en:Physical quantities",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "A measure of the density of a material. The ratio of the density of a substance to the density of water at the same temperature. It is dimensionless and therefore has no units."
      ],
      "id": "en-relative_density-en-noun-JCoYumm-",
      "links": [
        [
          "density",
          "density"
        ],
        [
          "temperature",
          "temperature"
        ],
        [
          "dimensionless",
          "dimensionless"
        ]
      ],
      "synonyms": [
        {
          "word": "specific gravity"
        }
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "word": "relative density"
}

{
  "forms": [
    {
      "form": "relative densities",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "relative density (countable and uncountable, plural relative densities)",
      "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",
        "English uncountable nouns",
        "en:Physical quantities"
      ],
      "glosses": [
        "A measure of the density of a material. The ratio of the density of a substance to the density of water at the same temperature. It is dimensionless and therefore has no units."
      ],
      "links": [
        [
          "density",
          "density"
        ],
        [
          "temperature",
          "temperature"
        ],
        [
          "dimensionless",
          "dimensionless"
        ]
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "specific gravity"
    }
  ],
  "word": "relative density"
}