"superior letter" meaning in English

{
  "forms": [
    {
      "form": "superior letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "superior letter (plural superior letters)",
      "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": "Wikipedia with redundant first parameter",
          "orig": "wikipedia with redundant first parameter",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "A letter written in superscript, usually when it indicates an abbreviation."
      ],
      "id": "en-superior_letter-en-noun-AtHC-zSI",
      "links": [
        [
          "superscript",
          "superscript"
        ],
        [
          "abbreviation",
          "abbreviation"
        ]
      ],
      "synonyms": [
        {
          "tags": [
            "obsolete"
          ],
          "word": "cock-up"
        }
      ],
      "wikipedia": [
        "superior letter"
      ]
    }
  ],
  "word": "superior letter"
}

{
  "forms": [
    {
      "form": "superior letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "superior letter (plural superior letters)",
      "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",
        "wikipedia with redundant first parameter"
      ],
      "glosses": [
        "A letter written in superscript, usually when it indicates an abbreviation."
      ],
      "links": [
        [
          "superscript",
          "superscript"
        ],
        [
          "abbreviation",
          "abbreviation"
        ]
      ],
      "wikipedia": [
        "superior letter"
      ]
    }
  ],
  "synonyms": [
    {
      "tags": [
        "obsolete"
      ],
      "word": "cock-up"
    }
  ],
  "word": "superior letter"
}