IOU stands for I owe you and we pronounce each letter separately. But how do we classify that construction"?
- abbreviation: a shortened form of a word or phrase
- acronym: an abbreviation formed from the initial letters of other words and pronounced as a word
- Initialism: an abbreviation consisting of initial letters pronounced separately
- back-formation: a word that is formed from an existing word which looks as though it is a derivative, typically by removal of a suffix
All definitions provided by Oxford Dictionaries Online
It can't be an abbreviation because there is no shortening, clipping or back-formation. Take for example phone which is an abbreviation of telephone, or edit which is a back-formation of editorship and editor. I would argue that abbreviations are words that have been shortened, a faster way of writing or saying something. Another example would be Prof for professor.
It can't be an acronym because we don't pronounce IOU as one word, whereas we do with NATO and RAM.
It can't be an initialism because if it was, it should be written as IOY (I Owe You)
Other examples that spring to mind is CU for see you and YRU for why are you, where initialism would dictate that the proper forms be SY and WAY.
How do linguists define this structure? Is there a more specific term than abbreviation?