CISO is a format used to store disk images. This is a file that is identical to the ISO format, but only stores the data needed to restore the disk. It is a compressed format that also allows you to encrypt your data with passwords.

CISO files are mainly associated with Ninetendoo Wii game disc images. They can also be used by this console's emulators. Wii Backup Fusion is often used to handle these files.