A portmanteau word is similar to a contraction, but contractions are formed from words that would otherwise appear together in sequence, such as do and not to make don't, whereas a portmanteau is formed by combining two or more existing words that all relate to a single concept.

https://en.wikipedia.org/wiki/Portmanteau

"Motel" and "fog" are portmanteaus, because you wouldn't put "motor hotel" or "smoke fog" in sentences that accept the accretions, whereas you could swap "prion" for "protein infection".