What is a file which cannot be modified or deleted?

A file that cannot be modified or deleted is often referred to as a "read-only" file. This type of file is set with permissions that restrict users from making any changes or removing it from the system. Read-only files are commonly used for important documents or system files to prevent accidental alterations or deletions. In some contexts, files may also be made immutable at the filesystem level, further enhancing their protection.