What is "nonnet"

A Nonnet is a type of poem which has the following requirements:

• It has nine lines
• The first line has 9 syllables, the second 8 syllables, the third 7 syllables until the ninth line which has one syllable.
• Has an iambic meter (stress every other syllable)

For e.g. Akhil Katyal's poem 'How to write a Nonnet!':

Hear hear, this is what a nonnet is
begin with 9, then with each line,
cut a syllable every time
by the 4th line, it's 6
by the 5th, it's 5
and as you dive
my hon', your
nonnet's
done!