A portmanteau word is a combination of two words that morph into one new word. Typically it combines both sounds and meanings. For example, 'smog' is a portmanteau, it is a combination of smoke and fog.
Portmanteaus became very famous from the story of “Through the Looking Glass.” It was the first time that the word portmanteau was used in the new sense. In the book, Humpty Dumpty explains to Alice how words are melded in the poem "Jabberwocky."
Interestingly, the word portmanteau itself is a portmanteau. It comes from the French porter + manteau which mean To Carry a Cloak. Before Through the Looking Glass, a portmanteau referred to a suitcase in English.