"SI joint" meaning in English

{
  "forms": [
    {
      "form": "SI joints",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "SI joint (plural SI joints)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "sacroiliac joint"
        }
      ],
      "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 topic categories using raw markup",
          "parents": [
            "Entries with topic categories using raw markup",
            "Entry maintenance"
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          "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"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Anatomy",
          "orig": "en:Anatomy",
          "parents": [
            "Biology",
            "Medicine",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Abbreviation of sacroiliac joint."
      ],
      "id": "en-SI_joint-en-noun-qL5YuWoP",
      "links": [
        [
          "sacroiliac joint",
          "sacroiliac joint#English"
        ]
      ],
      "synonyms": [
        {
          "word": "SIJ"
        }
      ],
      "tags": [
        "abbreviation",
        "alt-of"
      ]
    }
  ],
  "word": "SI joint"
}

{
  "forms": [
    {
      "form": "SI joints",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "SI joint (plural SI joints)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "sacroiliac joint"
        }
      ],
      "categories": [
        "English abbreviations",
        "English countable nouns",
        "English entries with incorrect language header",
        "English entries with topic categories using raw markup",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with non-redundant non-automated sortkeys",
        "en:Anatomy"
      ],
      "glosses": [
        "Abbreviation of sacroiliac joint."
      ],
      "links": [
        [
          "sacroiliac joint",
          "sacroiliac joint#English"
        ]
      ],
      "tags": [
        "abbreviation",
        "alt-of"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "SIJ"
    }
  ],
  "word": "SI joint"
}