"empty base" meaning in English

{
  "forms": [
    {
      "form": "empty bases",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "empty base (plural empty bases)",
      "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": "topical",
          "langcode": "en",
          "name": "Baseball",
          "orig": "en:Baseball",
          "parents": [
            "Ball games",
            "Sports",
            "Human activity",
            "Human behaviour",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "They have to pitch to the slugger because there are no empty bases."
        }
      ],
      "glosses": [
        "A base without a runner on it."
      ],
      "id": "en-empty_base-en-noun-r6H6C7ms",
      "links": [
        [
          "baseball",
          "baseball"
        ]
      ],
      "raw_glosses": [
        "(baseball) A base without a runner on it."
      ],
      "topics": [
        "ball-games",
        "baseball",
        "games",
        "hobbies",
        "lifestyle",
        "sports"
      ]
    }
  ],
  "word": "empty base"
}

{
  "forms": [
    {
      "form": "empty bases",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "empty base (plural empty bases)",
      "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",
        "en:Baseball"
      ],
      "examples": [
        {
          "text": "They have to pitch to the slugger because there are no empty bases."
        }
      ],
      "glosses": [
        "A base without a runner on it."
      ],
      "links": [
        [
          "baseball",
          "baseball"
        ]
      ],
      "raw_glosses": [
        "(baseball) A base without a runner on it."
      ],
      "topics": [
        "ball-games",
        "baseball",
        "games",
        "hobbies",
        "lifestyle",
        "sports"
      ]
    }
  ],
  "word": "empty base"
}