The term for a word made by blending parts of two other words, such as “smog” from “smoke” and “fog,” is portmanteau. In linguistics, portmanteau is also called a blend word or lexical blend.
The word portmanteau originally referred to a two-compartment traveling case. Lewis Carroll gave the term its linguistic sense in Through the Looking-Glass, published in 1871, when Humpty Dumpty describes words whose meanings and sounds are combined. The metaphor suggests two ideas packed together in one word.
“Smog” combines smoke and fog, while “motel” combines motor and hotel. Other familiar examples include brunch from breakfast and lunch, and infomercial from information and commercial. Blends usually preserve recognizable pieces of their source words, though the amount taken from each can vary.
A blend is not exactly the same as a compound. A compound joins whole words or largely complete forms, as in “toothbrush,” whereas a blend compresses or overlaps them. “Neologism” is broader: it means a newly coined word, whether or not it was formed by blending.