noun
pen pal; pen friend
A friend one exchanges letters with, usually in a different country, for cultural exchange or language practice.
中学生のとき、イギリスにペンフレンドがいた。
When I was in junior high, I had a pen pal in England.
ペンフレンドと文通を続けている。
I keep up a correspondence with my pen pal.
文通 is the act of exchanging letters, while ペンフレンド refers to the person you exchange letters with.
Loanword from English 'pen friend', adapted into Japanese as ペンフレンド.