What is the term for a word formed by merging two words, such as 'brunch'?

A word formed by merging two words, such as brunch, is called a portmanteau. Brunch combines breakfast and lunch, preserving parts of both words and blending their meanings.

In linguistics, portmanteau is also called a blend or blend word. Other familiar examples include smog, from smoke and fog, and motel, from motor or motorist and hotel. These formations usually become popular because they name a new idea, activity, or product more economically than a phrase would.

The term comes from French portemanteau, originally a two-compartment traveling case or clothes carrier. Lewis Carroll helped give the word its modern literary sense in Through the Looking-Glass, published in 1871. In the book, Humpty Dumpty uses it to explain words that carry two meanings packed together.

A portmanteau is not exactly the same as a compound word. A compound joins complete words, as in toothbrush, while a blend usually clips and fuses portions of its source words. An acronym, by contrast, is built from initial letters, and a contraction normally shortens a single word or phrase without creating this kind of lexical blend.