What do you call words that are formed by combining two existing words?

The concept of combining two existing words to form a new one is most accurately described as "compounds." Compounding is a common linguistic process where two standalone words come together to create a new word that often carries a meaning related to both original terms. For example, the words "tooth" and "brush" combine to form "toothbrush."

While "blended words" refers to a different process where parts of two or more words are merged to create a new term, such as "smog" from "smoke" and "fog," it does not encompass the full definition of combining complete existing words. In contrast to compound words, blended words often take segments from multiple words instead of the entire word.

Backformation, on the other hand, involves creating a new word by removing a suffix or prefix from an existing word rather than combining two whole words.

Thus, the correct terminology for combining two existing words is accurately identified as "compounds."