What does NVL stand for?

Quite simply: Null VaLue

The function substitutes any NULLs in a given resultset column with the value given as the second parameter.

"Null Value Logic" according to http://www.abbreviations.com/term/157605, but most of the other references I have found support Phil's "Null Value" supposition. I haven't found a definitive origin.

In assembly language programming also programmers used NVL for Null Value Logic. So in this case also it indicates Null Value.