I learned an interesting new word from the radio yesterday – sniglet – which is defined as “any word that doesn’t appear in the dictionary, but should”.

It was apparently popularized by the comedian/actor Rich Hall while he was working on Not Necessarily the News, an HBO comedy series from the 1980s, who has also published a number of collections of sniglets [source].

Here are some collections of sniglets:
http://bertc.com/subfour/truth/sniglets.htm
http://grammar.about.com/od/rs/g/Sniglet-term.htm

A related term is:

– nonce word – a word coined or used for a special occasion [source]