"zero-day attack" meaning in English

{
  "etymology_text": ": zero-day attack < → δείτε τις λέξεις zero-day και attack",
  "forms": [
    {
      "form": "zero-day attack"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Διαδίκτυο (αγγλικά)",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Πληροφορική (αγγλικά)",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "βλ. συνώνυμο zero-day exploit"
      ],
      "id": "el-zero-day_attack-en-noun-BrnL6zKR",
      "raw_tags": [
        "πληροφορική",
        "διαδίκτυο"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "zero-day exploit"
    }
  ],
  "word": "zero-day attack"
}

{
  "etymology_text": ": zero-day attack < → δείτε τις λέξεις zero-day και attack",
  "forms": [
    {
      "form": "zero-day attack"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "Διαδίκτυο (αγγλικά)",
        "Πληροφορική (αγγλικά)"
      ],
      "glosses": [
        "βλ. συνώνυμο zero-day exploit"
      ],
      "raw_tags": [
        "πληροφορική",
        "διαδίκτυο"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "zero-day exploit"
    }
  ],
  "word": "zero-day attack"
}