A word made by blending parts of two words, such as “smog” from “smoke” and “fog,” is called a portmanteau.
In linguistics, portmanteau is a traditional term for what is also called a blend word or lexical blend. The process combines portions of the sounds or spellings of two words while often combining their meanings as well. “Smog” preserves the beginning of smoke and the ending of fog, creating a compact word for polluted air.
The literary history of the term goes back to Lewis Carroll. In Through the Looking-Glass, published in 1871, Humpty Dumpty describes words carrying two meanings at once as being like a portmanteau, an old two-compartment travel case. Linguists often prefer the plainer term blend, but portmanteau remains the standard quiz answer.
Portmanteaus differ from ordinary compounds, which usually preserve complete stems, as in blackboard or sunflower. Other well-known blends include motel from motor and hotel, brunch from breakfast and lunch, and Tanzania from Tanganyika and Zanzibar. A blend may begin as a novelty but become a permanent part of the vocabulary.