current-prefix-arg

current-prefix-arg is a variable defined in `C source code'.
Its value is

nil

Documentation:
The value of the prefix argument for this editing command.
It may be a number, or the symbol `-' for just a minus sign as arg,
or a list whose car is a number for just one or more C-u's
or nil if no argument has been specified.
This is what `(interactive "P")' returns.