A neologism is a new word or expression, sometimes a pejorative, that's in everyday usage, such as "staycation;" it could also refer to a familiar, or an established, word or expression being used in a new sense, such as "spam." For example, "digital detox" consists of two words that most people are already well aware of, but they aren't used to seeing them together; this new combination comes with a new meaning.
For years, most people were only familiar with Spam as a brand of food. Now, it's a widely known neologism that means unwanted messages. There's even a neologism for the act of staying home during vacation, known as "staycation."
In psychology, a neologism is a word that a schizophrenic, or another psychotic, might make up, so the word only really makes sense or has meaning to that individual. Borrowing from that, a neologism is generally invented to be clever or funny, perhaps just to a few who will understand the humor behind it. Take, for example, the neologism "palingerism," which the "Huffington Post" defines as "the deliberate attempt of Ivy League ghostwriters to sound homey and folksy by putting words into the mouth of someone who barely understands them." This was a reference to Sarah Palin.