[We have public-domain dictionaries available to us; can we please provide one of those definitions without an link to M-W?]
The Merriam-Webster definition of portmanteau gives two meanings:
- a large suitcase
- a word or morpheme whose form and meaning are derived from a blending of two or more distinct forms (as smog from smoke and fog)
- See Portmanteau Words By Geoff Nunberg for an interesting commentary on this subject.