The term 'FORMAL LETTER OPENING' can be summarized as a four-letter acronym, SIRS. This acronym is used to begin a formal letter and stands for 'Salutation, Introduction, Reason, and Salutation'. The salutation is the greeting at the beginning of the letter, such as 'Dear Sir/Madam'. The introduction is the first sentence of the letter, which explains the purpose of the letter. The reason is the body of the letter, which explains the details of the purpose. Finally, the salutation is the closing of the letter, such as 'SINCERELY'.