A nonce word is one coined for a particular occasion.
For instance, if you ever needed to communicate an intense need for chocolate it would be possible to speak of suffering from chocodrawal symptoms.
If you needed to express your displeasure at someone else's infant - or, indeed, your own - getting into your private possessions or your conversation then perhaps toddlecreep would cover it.
The trouble with nonce words is, of course, that if they're actually any good then they'll be used again and again and soon stop being, well, nonce words, at all.
One other reason nonce words come into being is for use in language experiments.
So let's do one now: which of these creatures is a flooble?
photo by Robert
photo by NIAID
Well, don't ask me: flooble is a nonce word. I just made it up.*
Words To Use Today: nonce word. This phrase comes from for the nonce, which is a mangling of the phrase for then anes, which in the 1100s meant for the once.
*But most people would probably say the first, all the same: rounded sounds tends to suggest a rounded body in most people's minds.