A lune (rhymes with moon) is a very short poem. It has only three lines. It is similar to a haiku. A haiku has three lines, and it follows a 5/7/5 syllable pattern. The lune's syllable pattern is 5/3/5. Since the middle line is limited to three syllables, it is often the shortest line of the three. This makes a lune's right side curve a bit like a crescent moon.
Here is an example:
Books, papers, notes, pens
What a mess
Is piled on my desk