Fair point on "string," though i never really encountered it, i opened up an old goldfarb book, and it's not in the index, but it looks like it is used in my old enderton "mathematical introduction to logic," so, again, fair point. I think "statement" would be a better term, maybe.
enderton page 107, 289, etc: http://books.google.com/books?id=dVncCl_EtUkC&printsec=front...