pen pal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person with whom one becomes acquainted through a friendly, regular correspondence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person with whom one communicates using letters, usually over a long distance.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person you come to know by frequent friendly correspondence