letter
noun
ˈletə(r) 
ˈletər 
1 a message that is written down or printed on paper and usually put in an envelope and sent to somebodya business/thank-you, etc. lettera letter of complaint (British English) to post a letter (North American English) to mail a letterThere's a letter for you from your mother.You will be notified by letter.Help
You will find compounds ending in letter at their place in the alphabet.2 a written or printed sign representing a sound used in speech‘B’ is the second letter of the alphabet.Write your name in capital/block letters.3 (North American English) a sign in the shape of a letter that is sewn onto clothes to show that a person plays in a school or college sports teamIdioms
the letter of the law
(often disapproving) the exact words of a law or rule rather than its general meaningThey insist on sticking to the letter of the law.We might be guilty of slightly bending the letter of the law.to the letter
doing/following exactly what somebody/something says, paying attention to every detail
I followed your instructions to the letter.