"flat line" meaning in English

{
  "forms": [
    {
      "form": "flat lines",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "flat line (plural flat lines)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "flatline"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "2023 February 8, Sir Michael Holden, “Comment: Boom or bust: time to decide”, in RAIL, number 976, page 3",
          "text": "\"Overall satisfaction with rail journey\", as measured fortnightly by Transport Focus, has maintained a broadly flat line at around 85% across the last four months of travel disruption, while \"satisfaction with punctuality/reliabilty\" is hovering around 75%.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative form of flatline."
      ],
      "id": "en-flat_line-en-noun--9sim0Cb",
      "links": [
        [
          "flatline",
          "flatline#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "flat line"
}

{
  "forms": [
    {
      "form": "flat lines",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "flat line (plural flat lines)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "flatline"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "2023 February 8, Sir Michael Holden, “Comment: Boom or bust: time to decide”, in RAIL, number 976, page 3",
          "text": "\"Overall satisfaction with rail journey\", as measured fortnightly by Transport Focus, has maintained a broadly flat line at around 85% across the last four months of travel disruption, while \"satisfaction with punctuality/reliabilty\" is hovering around 75%.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative form of flatline."
      ],
      "links": [
        [
          "flatline",
          "flatline#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "flat line"
}