ispell-personal-dictionary

ispell-personal-dictionary is a variable defined in `ispell.el'.
Its value is

nil

Documentation:
File name of your personal spelling dictionary, or nil.
If nil, the default personal dictionary, ("~/.ispell_DICTNAME" for ispell or
"~/.aspell.LANG.pws" for aspell) is used, where DICTNAME is the name of your
default dictionary and LANG the two letter language code.

You can customize this variable.