1. <operating system> (Or "erase") To make a file inaccessible.
Usually this operation only deletes information from the tables the file system uses to locate named files; the file's contents still exist on disk and can sometimes be recovered by scanning the whole disk for strings which are known to have been in the file. Files created subsequently on the same disk are quite likely to reuse the same blocks and thus overwrite the deleted file's data permanently.
2. <character> The control character with ASCII code 127. Usually entering this character from the keyboard deletes the last character typed from the input buffer. Sadly there is great confusion between operating systems and keyboard manufacturers as to whether this function should be assigned to the delete or backspace key/character.
The choice of code 127 (binary 1111111) is not arbitrary but dates back to the use of paper tape for input. The delete key rewound the tape by one character and punched out all seven holes, thus obliterating whatever character was there before. The tape reading software ignored any delete characters in the input.