"retention tank" meaning in English

{
  "forms": [
    {
      "form": "retention tanks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "retention tank (plural retention tanks)",
      "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"
        }
      ],
      "examples": [
        {
          "ref": "2020 January 2, “Fleet News”, in Rail, page 20:",
          "text": "Eleven four-coach 'Castles' are on order, with eight delivered. These are former GWR Mk 3s that have been converted by fitting sliding doors, passenger information screens and retention tanks.",
          "type": "quote"
        }
      ],
      "glosses": [
        "A tank for the retention of toilet waste, e.g. on a train."
      ],
      "id": "en-retention_tank-en-noun-C2o5jE2V",
      "links": [
        [
          "tank",
          "tank"
        ],
        [
          "retention",
          "retention"
        ],
        [
          "toilet",
          "toilet"
        ],
        [
          "waste",
          "waste"
        ],
        [
          "train",
          "train"
        ]
      ]
    }
  ],
  "word": "retention tank"
}

{
  "forms": [
    {
      "form": "retention tanks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "retention tank (plural retention tanks)",
      "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",
        "English terms with quotations",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "ref": "2020 January 2, “Fleet News”, in Rail, page 20:",
          "text": "Eleven four-coach 'Castles' are on order, with eight delivered. These are former GWR Mk 3s that have been converted by fitting sliding doors, passenger information screens and retention tanks.",
          "type": "quote"
        }
      ],
      "glosses": [
        "A tank for the retention of toilet waste, e.g. on a train."
      ],
      "links": [
        [
          "tank",
          "tank"
        ],
        [
          "retention",
          "retention"
        ],
        [
          "toilet",
          "toilet"
        ],
        [
          "waste",
          "waste"
        ],
        [
          "train",
          "train"
        ]
      ]
    }
  ],
  "word": "retention tank"
}