"em-" meaning in French

{
  "head_templates": [
    {
      "args": {},
      "expansion": "em-",
      "name": "fr-prefix"
    }
  ],
  "lang": "French",
  "lang_code": "fr",
  "pos": "prefix",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "French entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "em- (form of en- before b, m or p)"
      ],
      "id": "en-em--fr-prefix-Gmulfuej",
      "links": [
        [
          "em-",
          "em-#English"
        ]
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/ɑ̃/"
    }
  ],
  "word": "em-"
}

{
  "head_templates": [
    {
      "args": {},
      "expansion": "em-",
      "name": "fr-prefix"
    }
  ],
  "lang": "French",
  "lang_code": "fr",
  "pos": "prefix",
  "senses": [
    {
      "categories": [
        "French 1-syllable words",
        "French entries with incorrect language header",
        "French lemmas",
        "French prefixes",
        "French terms with IPA pronunciation"
      ],
      "glosses": [
        "em- (form of en- before b, m or p)"
      ],
      "links": [
        [
          "em-",
          "em-#English"
        ]
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/ɑ̃/"
    }
  ],
  "word": "em-"
}