Deletion is the removal or erasure of data. This is often a permanent operation, with no chance to recover the data and restore it to its original location.

Deletion can operate at several levels in a computer. For example, when a character is deleted, it is removed from a file or erased from the display screen. A complete file can be deleted from a disk, as can a directory (and all the files in the directory).

The deletion of data is a necessary part of maintaining a computer. No matter how big a hard disk is, in terms of the amount of information it can accommodate, it will eventually fill up. Operating systems such as Windows come equipped with a cleanup program. Files identified as unnecessary can be deleted, as can temporarily-stored files. The latter include temporary Internet files, downloaded program files, and files which have been through the first step of a deletion process, having been removed from their original location and moved to the recycle bin (in Windows) or the trash can (Macintosh). Files in the waste area can be retrieved until another command to permanently delete them (purge) has been given.

Deletion algorithms also allow for the deletion of hierarchical data, be it a node in a data tree or data in a linked list. Deleting data serves to free up space on a computer's hard disk. This improves the efficiency of operation of the computer.