RTF is short for Rich Text Format. It is a way a computer saves a document so that some shape of the writing is kept — bold, italic, different sizes — without using a full Word file. Microsoft made it so that different programs could share text more easily. You know it by the ending .rtf. It is an old, quiet format now; most people send .docx or PDF instead, but a few offices and old machines still ask for it. Though actually, when an email box offers ‘rich text,’ that often means styled typing in the message, not always a true .rtf file. It is a computer word. You say the three letters.

RTF（豐富文字格式）