What is a word formed by blending two words, like 'brunch'?

A word formed by blending two words, like “brunch,” is called a portmanteau, or more technically a blend word. “Brunch” combines parts of “breakfast” and “lunch” while carrying a meaning related to both.

The literary use of “portmanteau” comes from Lewis Carroll’s Through the Looking-Glass, published in 1871. Humpty Dumpty compares a word containing two meanings with a portmanteau, a piece of luggage divided into two compartments. Carroll illustrated the idea with invented words from “Jabberwocky,” including “slithy,” from “slimy” and “lithe.”

Blends differ from ordinary compound words. “Starfish” is a compound because both source words remain intact, while “smog,” from “smoke” and “fog,” is a blend. Other familiar examples include “motel,” from “motor” and “hotel,” and “sitcom,” from “situation” and “comedy.”

“Portmanteau” and “blend” are often used interchangeably, although linguists may distinguish subtypes based on how much of each source word survives. “Acronym” and “initialism,” by contrast, are formed from initials rather than overlapping word fragments.