A neologism is a word someone's just invented.
They're all failures.
That's not to say we don't need neologisms. Of course we do. We couldn't manage without words like hashtag* and jeggings**, could we?
The trouble is that once a neologism becomes widely understood then it stops being a neologism and becomes just an ordinary word.
All words were neologisms once, after all!
Most new words that survive are those with guessable meanings. Hashtag and jeggings are both good examples of words that contain clues to what they mean. This makes them easier to remember. Being able to guess their pronunciation helps, too.
Many words, sadly, don't make it. For instance the charming and highly evocative boogrot***, meaning stomach-upset, has never become widely established.
But still, I've just used it here, so that's given the poor thing a little bit of extra life.
Word To Use Today: any neologism. This is easy, because you can make up your own.
The word neologism is from the Greek neo, which means new, and logos, which means word.
*A Twitter address, as far as I understand, which isn't very far at all (probably about three and a half centimetres).
** Leggings which are styled to look like jeans.
***Mr Boog used to claim responsibility for catering at Royal Holloway College in the 1970s.