Residual data is information that remains on storage after a workload ends, a file is deleted, or a volume is reused. The data may not be visible through normal applications, but it can still exist in unallocated blocks, cache pages, logs, temporary files, or partially overwritten sectors. In cloud and virtualized environments, this matters because storage is often reused across tenants or tasks.
From a security perspective, residual data creates data remanence risk: a new workload may recover artifacts that belonged to a previous one. Attackers look for this kind of leftover content to harvest metadata, configuration files, database fragments, or secrets. Defenders reduce the risk by zeroing reused blocks, securely wiping disks, encrypting storage, limiting sensitive data on ephemeral volumes, and testing cleanup paths to confirm that deleted data cannot be recovered.