Computerspeak for 'Zero', Null's usage derives from the mathematics "Of or relating to a set having no members or to zero magnitude."

Unix computers have a null device, located at /dev/null: if you try and read this you will get a null result, if you try and write to it your data will disappear. This has lead to a common quote in email: "Constructive replies to (email address), flames to /dev/null"

Abbreviated to NUL in ASCII.