"pbk" meaning in Anglais

{
  "categories": [
    {
      "kind": "other",
      "name": "Lemmes en anglais",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Noms communs en anglais",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Anglais",
      "orig": "anglais",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Anglais",
  "lang_code": "en",
  "pos": "noun",
  "pos_title": "Nom commun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en anglais de la bibliothéconomie",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Lexique en anglais de l’édition",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "Exemple : ISBN 0-8061-3038-5 (pbk.)"
        }
      ],
      "glosses": [
        "Abréviation de paperback, c’est-à-dire couverture molle."
      ],
      "id": "fr-pbk-en-noun-alb0iRC9",
      "raw_tags": [
        "Bibliothéconomie"
      ],
      "topics": [
        "editing"
      ]
    }
  ],
  "word": "pbk"
}

{
  "categories": [
    "Lemmes en anglais",
    "Noms communs en anglais",
    "anglais"
  ],
  "lang": "Anglais",
  "lang_code": "en",
  "pos": "noun",
  "pos_title": "Nom commun",
  "senses": [
    {
      "categories": [
        "Lexique en anglais de la bibliothéconomie",
        "Lexique en anglais de l’édition"
      ],
      "examples": [
        {
          "text": "Exemple : ISBN 0-8061-3038-5 (pbk.)"
        }
      ],
      "glosses": [
        "Abréviation de paperback, c’est-à-dire couverture molle."
      ],
      "raw_tags": [
        "Bibliothéconomie"
      ],
      "topics": [
        "editing"
      ]
    }
  ],
  "word": "pbk"
}