chain letter

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

• noun A letter sent to a number of people asking each recipient to send copies with the same request to a specified number of others. The circulation of a chain letter increases in geometrical progression as long as the instructions are followed by all recipients.

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

• noun A letter that is mailed successively to different recipients, with each recipient being instructed in the body of the letter to mail the letter to another person or persons.

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

• noun a letter that is sent successively to several people