A portmanteau word (/pɔːrtˈmæntoʊ/ (ⓘ)) is made of other words, or parts of words. It comes from putting two (or more) other words together, like motel, from motor and hotel.
Portmanteau originally meant a type of suitcase with two separate parts for storage. The first use as a word combination appeared in Lewis Carroll's book Through the Looking-Glass. There Humpty Dumpty explains that some of the unusual words in Jabberwocky are other words packed together into one word, as two parts of a portmanteau suitcase are packed together.
Portmanteau itself is an example, combining the French words for carry (porter) and coat (manteau) into a coat carrier. Wikipedia is a portmanteau of wiki (Hawaiian for fast) and encyclopedia.