What is the longest abbreviation?

There's no single "longest abbreviation" because it depends on what we consider an abbreviation.

Here are some examples of different types of abbreviations, and their potential lengths:

* Acronyms: These are formed by taking the first letters of words and making a new word. They can be quite long, like "World Health Organization" (WHO), which is just 3 letters. There's no limit on how long an acronym can be.

* Initialisms: Similar to acronyms, but the letters are pronounced individually. For example, "United States of America" (USA). Again, there's no limit on length.

* Shortened forms: These are words or phrases with parts removed, like "Professor" or "Dr." These can be shorter than acronyms, but some can be quite long depending on the original word or phrase.

Ultimately, the "longest abbreviation" depends on what you're looking for. If you're looking for the longest acronym, then "International Organization for Standardization" (ISO) might be a contender at 3 letters. If you're looking for a long shortened form, something like "H.M.S. Victory" (for Her Majesty's Ship Victory) could be considered.

Let me know if you have a specific type of abbreviation in mind, and I can help you find some examples!