a standardized letter that can be sent to any number of persons, occasionally personalized by inserting the name of each recipient in the salutation.
Origin of form letter
An Americanism dating back to 1905–10
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for form letter
a single copy of a letter that has been mechanically reproduced in large numbers for circulation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012