The word 'letter' has a wide range of use in the English language. There are a variety of letters. Any written (also printed) communication from one person to another to convey information, ideas, advice or opinion is a letter. This forms the bulk of the material handled by the Postal Department. The writers of these letters are from all parts of the world.
Sharma, Mohan Lal
"All Writers Not Men of Letters,"
Word Ways: Vol. 5
, Article 19.
Available at: https://digitalcommons.butler.edu/wordways/vol5/iss2/19