An abbreviation formed from the initial letters of other words and pronounced as a word is known as what?

An abbreviation created from the initial letters of a group of words and pronounced as a single word is called an acronym. This term is specifically used to describe instances where, rather than just shortening the phrase, the initial letters combine to form a new, pronounceable term, such as "NASA" (National Aeronautics and Space Administration) or "RADAR" (Radio Detection and Ranging). Acronyms are a useful linguistic tool, as they often facilitate communication by shortening lengthy phrases or titles into manageable terms that can be easily spoken and remembered.

In contrast, other terms listed do not fit this definition. A word root refers to the most basic part of a word, to which prefixes and suffixes can be added. Alliteration involves the repetition of consonant sounds at the beginning of words in a sequence, which is a stylistic device rather than an abbreviation. A topic is the subject of a discussion or piece of writing, and it does not relate to the formation of words from initials. Thus, the most accurate identification for the definition provided in the question is an acronym.