Real Dictionary

pen pal
countable noun: A pen pal is someone you write friendly letters to and receive letters from, although the two of you may never have met.
noun: a person with whom one regularly exchanges letters, often a person in another country whom one has not met

pen pal in American English
a person with whom one keeps up an exchange of letters, usually someone so far away that a personal meeting is unlikely
a person, esp. a stranger in another country, with whom one arranges a regular exchange of letters