"Snellen chart" meaning in English

{
  "etymology_text": "Named after the Dutch ophthalmologist Hermann Snellen, who developed the chart in 1862.",
  "forms": [
    {
      "form": "Snellen charts",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "Snellen chart (plural Snellen charts)",
      "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": "English entries with language name categories using raw markup",
          "parents": [
            "Entries with language name categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English terms with non-redundant non-automated sortkeys",
          "parents": [
            "Terms with non-redundant non-automated sortkeys",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A chart bearing letters or other symbols, used to test visual acuity."
      ],
      "id": "en-Snellen_chart-en-noun-kBE5o1RI",
      "links": [
        [
          "chart",
          "chart"
        ],
        [
          "letter",
          "letter"
        ],
        [
          "symbol",
          "symbol"
        ],
        [
          "visual",
          "visual"
        ],
        [
          "acuity",
          "acuity"
        ]
      ],
      "synonyms": [
        {
          "word": "test card"
        }
      ]
    }
  ],
  "word": "Snellen chart"
}

{
  "etymology_text": "Named after the Dutch ophthalmologist Hermann Snellen, who developed the chart in 1862.",
  "forms": [
    {
      "form": "Snellen charts",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "Snellen chart (plural Snellen charts)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English entries with language name categories using raw markup",
        "English eponyms",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with non-redundant non-automated sortkeys"
      ],
      "glosses": [
        "A chart bearing letters or other symbols, used to test visual acuity."
      ],
      "links": [
        [
          "chart",
          "chart"
        ],
        [
          "letter",
          "letter"
        ],
        [
          "symbol",
          "symbol"
        ],
        [
          "visual",
          "visual"
        ],
        [
          "acuity",
          "acuity"
        ]
      ]
    }
  ],
  "synonyms": [
    {
      "word": "test card"
    }
  ],
  "word": "Snellen chart"
}