A nonce-word is a word created for a single occasion or a very limited use, often to fit a particular poem, sound pattern, or expressive need. A poet may invent a nonce-word to capture a feeling, imitate a voice, or add humor, texture, or musicality. Unlike a word that becomes established in the language, a nonce-word may never be used again.
Example
The moon went silver-blink across the hill,
while night made hush and hushen all things still.