What is the name for a word formed by combining two words, like 'brunch'?

A word formed by blending two words, such as “brunch” from breakfast and lunch, is a portmanteau.

Unlike a straightforward compound word, a portmanteau usually fuses portions of the source words and combines their meanings. “Smog” blends smoke and fog, while “motel” combines motor or motorist with hotel. The result may preserve recognizable pieces of both originals, but it does not need to keep either word intact.

The literary use of “portmanteau” comes from Lewis Carroll’s Through the Looking-Glass, published in 1871. Carroll used the term metaphorically for words packed together, borrowing the name of a two-compartment traveling case. The word “brunch” itself was proposed by British writer Guy Beringer in an 1895 essay and was later popularized in print.

Many portmanteaus eventually become ordinary vocabulary, so speakers may forget that they were blends. “Neologism” simply means a newly created or newly adopted word, and some neologisms are portmanteaus. An acronym is built from initials, making it a different word-formation process.