An air gap is the separation of a backup or system from any network so it cannot be reached online. It can be physical, such as a tape stored outside the data center, or logical, such as storage that only connects during the backup window. That way, ransomware that encrypts the network cannot reach the isolated copy.
Go deeper on the TecnetBlog: How to Protect Your Backups Against Ransomware
air-gapped backup, offline backup, logical air gap, isolated backup