A person with whom one becomes friendly by exchanging letters, especially someone in a foreign country whom one has never met.
or in other defenition:
pen-friend is someone you write friendly letters to and receive letters from, although the two of you may never have met.
penfriend is also called pen pal or penpal friend. It is not like normal friendships that starts by visiting each other. Pen friends usually send each other some messages via different ways and make their friendship anonymously .