"fold flat" meaning in English

{
  "forms": [
    {
      "form": "fold flats",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "fold flat (plural fold flats)",
      "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"
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          "source": "w"
        },
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          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
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          "kind": "other",
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          "parents": [],
          "source": "w"
        }
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      "examples": [
        {
          "ref": "2007 September, Hadjimichaela et al., “An Alignment and Integration Technique for Mirror Segment Pairs on the Constellation X Telescope”, in Proceedings of SPIE, volume 6688, →DOI, retrieved 2021-12-27, pages 66881D-4:",
          "text": "By reflecting the incident beam off of a rotating fold flat located on the optical axis of mirror segment..., we can conveniently image a range of azimuthal positions.",
          "type": "quote"
        }
      ],
      "glosses": [
        "A fold mirror that is a \"flat\", i.e. a plane mirror"
      ],
      "id": "en-fold_flat-en-noun-~pZ-T4G-",
      "links": [
        [
          "fold mirror",
          "fold mirror"
        ],
        [
          "plane mirror",
          "plane mirror"
        ]
      ],
      "related": [
        {
          "word": "fold sphere"
        }
      ]
    }
  ],
  "word": "fold flat"
}

{
  "forms": [
    {
      "form": "fold flats",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "fold flat (plural fold flats)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "fold sphere"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
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      "examples": [
        {
          "ref": "2007 September, Hadjimichaela et al., “An Alignment and Integration Technique for Mirror Segment Pairs on the Constellation X Telescope”, in Proceedings of SPIE, volume 6688, →DOI, retrieved 2021-12-27, pages 66881D-4:",
          "text": "By reflecting the incident beam off of a rotating fold flat located on the optical axis of mirror segment..., we can conveniently image a range of azimuthal positions.",
          "type": "quote"
        }
      ],
      "glosses": [
        "A fold mirror that is a \"flat\", i.e. a plane mirror"
      ],
      "links": [
        [
          "fold mirror",
          "fold mirror"
        ],
        [
          "plane mirror",
          "plane mirror"
        ]
      ]
    }
  ],
  "word": "fold flat"
}