letter  
 noun [ C ] uk/ˈlet.ər/ us/ˈlet̬.ɚ/
  (MESSAGE)      
 A1 信,信件
a written message from one person to another, usually put in an envelope and sent by post
  I got a letter from the bank this morning. 今天早晨我收到了一封银行的来函。 
 - More examples
 - He read her letter aloud to the rest of the family.
 - I folded the letter in half and put it in an envelope.
 - We try to answer readers' letters as promptly as we can.
 - In those days people used to write a lot more letters.
 - I had a lovely, long, chatty letter from my friend this morning.
 
  (SYMBOL)        
A2 字母
any of the set of symbols used to write a language, representing a sound in the language
  the letter D 字母D 
 the letter of the lawformal 法律的准确字面意义
the exact words of the law and not its more important general meaning
 - More examples
 - What letter does it start with?
 - How many letters are there in the alphabet?
 - Z is the last letter of the alphabet.
 - Our names all start with the letter P.
 - Letter S is the longest letter in the English dictionary.
 
Idiom   
to the letteridiom  一丝不苟地;丝毫不差地
If you obey instructions or rules to the letter, you do exactly what you have been told to do, giving great attention to every detail.
  I followed the instructions to the letter and it still went wrong. 我严格按指令行事,但还是出了问题。 
 letter       
adjective [ before noun ] US uk/ˈlet.ər/ us/ˈlet̬.ɚ/
 (美国)标准信纸(长11英寸,宽8.5英寸)
used to refer to a standard size of paper in the US, 8.5 inches by 11 inches
  Fliers are usually printed either on letter size or legal size paper. 印传单的纸一般是标准信纸或法律规格纸。