A character created when alt+255(numeric) is entered at the DOS prompt. Part of the Extended Ascii Character Set, alt 255 is an nonprinting null character. Because it is unprinted, it appears to be a space.

Ascii Code: 255 Hex Code: FF

(An old DOS trick was to use this character to put "spaces" in filenames. These files were subsequently difficult for the average user to delete, because they would try to put a space in the filename to be delteted, thus breaking command.com's command parse.)