undelete is a process whereby the recovery of deleted files is made possible under a Disk Operating System (eg MSDOS). When a file is deleted under a DOS, it's entry in the directory table for that particular directory has the first character of it's filename replaced with ASCII character #255. So essentialy the file that has been deleted is still there, and a fat chain still exists, and all information is still inside the directory table, it's just that the file is considered non-existant by the DOS.

Therefore it is possible to recover deleted files, provided the cluster/fat cluster entry hasn't been assigned to another file, or failing that, parts of the file. The undelete program simply searches for entries in a directory table that have been marked with ASCII character #255, and allows you to 'undelete' them by replacing the first character with a character you specify, of course you need to know what the first character of the filename was.

So when someone tells you they deleted a file in a DOS, and can't get it back, tell them to think again!