"light hydrogen" meaning in English

{
  "etymology_text": "So named because it is the isotope of hydrogen with the lightest atomic weight and atomic mass, having one nucleon (a single proton and no neutrons).",
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "light hydrogen (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English retronyms",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Hydrogen",
          "orig": "en:Hydrogen",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Isotopes",
          "orig": "en:Isotopes",
          "parents": [],
          "source": "w"
        }
      ],
      "coordinate_terms": [
        {
          "word": "heavy hydrogen"
        }
      ],
      "glosses": [
        "Synonym of protium (“\"the usual isotope of hydrogen\"”)."
      ],
      "id": "en-light_hydrogen-en-noun-en:Q12830437",
      "links": [
        [
          "protium",
          "protium#English:_Q12830437"
        ]
      ],
      "related": [
        {
          "word": "light water"
        },
        {
          "word": "light water reactor"
        }
      ],
      "senseid": [
        "en:Q12830437"
      ],
      "synonyms": [
        {
          "extra": "“\"the usual isotope of hydrogen\"”",
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "protium"
        }
      ],
      "tags": [
        "uncountable"
      ],
      "wikidata": [
        "Q12830437"
      ]
    }
  ],
  "word": "light hydrogen"
}

{
  "etymology_text": "So named because it is the isotope of hydrogen with the lightest atomic weight and atomic mass, having one nucleon (a single proton and no neutrons).",
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "light hydrogen (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "light water"
    },
    {
      "word": "light water reactor"
    }
  ],
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English retronyms",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Hydrogen",
        "en:Isotopes"
      ],
      "coordinate_terms": [
        {
          "word": "heavy hydrogen"
        }
      ],
      "glosses": [
        "Synonym of protium (“\"the usual isotope of hydrogen\"”)."
      ],
      "links": [
        [
          "protium",
          "protium#English:_Q12830437"
        ]
      ],
      "senseid": [
        "en:Q12830437"
      ],
      "synonyms": [
        {
          "extra": "“\"the usual isotope of hydrogen\"”",
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "protium"
        }
      ],
      "tags": [
        "uncountable"
      ],
      "wikidata": [
        "Q12830437"
      ]
    }
  ],
  "word": "light hydrogen"
}