"actinium-K" meaning in English

{
  "etymology_text": "From the former practice of naming the steps in a decay chain sequential letters. Francium was first identified in the naturally occurring actinium series.",
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "actinium-K (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": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Obsolete element names",
          "orig": "en:Obsolete element names",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Synonym of francium."
      ],
      "id": "en-actinium-K-en-noun-3boPjgsE",
      "links": [
        [
          "francium",
          "francium#English"
        ]
      ],
      "raw_glosses": [
        "(obsolete or historical) Synonym of francium."
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "francium"
        },
        {
          "word": "actinium K"
        }
      ],
      "tags": [
        "historical",
        "obsolete",
        "uncountable"
      ],
      "wikipedia": [
        "Decay chain#Actinium series"
      ]
    }
  ],
  "word": "actinium-K"
}

{
  "etymology_text": "From the former practice of naming the steps in a decay chain sequential letters. Francium was first identified in the naturally occurring actinium series.",
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "actinium-K (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with historical senses",
        "English terms with obsolete senses",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Obsolete element names"
      ],
      "glosses": [
        "Synonym of francium."
      ],
      "links": [
        [
          "francium",
          "francium#English"
        ]
      ],
      "raw_glosses": [
        "(obsolete or historical) Synonym of francium."
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "francium"
        }
      ],
      "tags": [
        "historical",
        "obsolete",
        "uncountable"
      ],
      "wikipedia": [
        "Decay chain#Actinium series"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "actinium K"
    }
  ],
  "word": "actinium-K"
}