1. A case or bag for carrying clothing and other necessaries when traveling; originally, of a form suitable for carrying on horseback; now applied to an oblong stiff leather case, which opens like a book, with hinges in the middle of the back: Leonard packed his portmanteau to carry on the train when he was traveling.
2. A combination of word parts to create a new word that is made up of the blended sounds of two other distinct words and combining the meanings of both of them: "Smog" and "motel" are examples of linguistic portmanteaus with "smog" meaning smoke + fog and "motel" meaning motor + hotel.
3. Etymology: from French porter, "carry" + manteau, "cloak, mantle".

See these additional examples of Portmanteau words.