orthography - Is IOU an abbreviation, an acronym, or an initialism?

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?

Answer

It could be characterized as a rebus

a riddle or puzzle made up of letters, pictures, or symbols whose names sound like the parts or syllables of a word or phrase [Merriam-Webster]

While a rebus often contains images, letters being used to represent syllables is common.

[Wikipedia]

In particular, the Encyclopaedia Britannica states

Literary rebuses use letters, numbers, musical notes, or specially placed words to make sentences. Complex rebuses combine pictures and letters. Rebuses may convey direct meanings, especially to inform or instruct illiterate people; or they may deliberately conceal meanings, to inform only the initiated or to puzzle and amuse.

....

A familiar English rebus is the debtor’s “IOU,” for “I owe you.”

If you wanted to be more precise in defining it, you could say alphabetic rebus.