"solar letter" meaning in English

{
  "antonyms": [
    {
      "word": "moon letter"
    }
  ],
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "ar",
        "3": "حَرْف شَمْسِيّ"
      },
      "expansion": "Calque of Arabic حَرْف شَمْسِيّ (ḥarf šamsiyy)",
      "name": "calque"
    }
  ],
  "etymology_text": "Calque of Arabic حَرْف شَمْسِيّ (ḥarf šamsiyy).",
  "forms": [
    {
      "form": "solar letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "solar letter (plural solar letters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
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        {
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          "name": "English entries with incorrect language header",
          "parents": [
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          "parents": [
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        {
          "kind": "topical",
          "langcode": "en",
          "name": "Linguistics",
          "orig": "en:Linguistics",
          "parents": [
            "Language",
            "Social sciences",
            "Communication",
            "Sciences",
            "Society",
            "All topics",
            "Fundamental"
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          "source": "w"
        }
      ],
      "glosses": [
        "Synonym of sun letter"
      ],
      "id": "en-solar_letter-en-noun-l82lycLI",
      "links": [
        [
          "sun letter",
          "sun letter#English"
        ]
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "sun letter"
        }
      ],
      "wikipedia": [
        "Sun letters"
      ]
    }
  ],
  "word": "solar letter"
}

{
  "antonyms": [
    {
      "word": "moon letter"
    }
  ],
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "ar",
        "3": "حَرْف شَمْسِيّ"
      },
      "expansion": "Calque of Arabic حَرْف شَمْسِيّ (ḥarf šamsiyy)",
      "name": "calque"
    }
  ],
  "etymology_text": "Calque of Arabic حَرْف شَمْسِيّ (ḥarf šamsiyy).",
  "forms": [
    {
      "form": "solar letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "solar letter (plural solar letters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
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        "English countable nouns",
        "English entries with incorrect language header",
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        "en:Linguistics"
      ],
      "glosses": [
        "Synonym of sun letter"
      ],
      "links": [
        [
          "sun letter",
          "sun letter#English"
        ]
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "sun letter"
        }
      ],
      "wikipedia": [
        "Sun letters"
      ]
    }
  ],
  "word": "solar letter"
}