Short for a multi-syllabic rhyme. A multi is a multiple syllable rhyme. There are several different types of multis but traditionally a multi is the break down of a multiple syllable word with individual words that rhyme with each syllable.
Like a black man when I *attack*
You'll feel the point like *a tack*
Attack is pronounced Uh-Tack, and then it's multiple syllables are broken down into rhyming individual syllables (doesn't nessicarly have to be one syllable, you could rhyme a part of a word to match that syllable).
Another weaker version of a multi is simply rhyming two or more words with two or more words. No break down, meaning the words them selfs rhyme with each other on their own.
"Villain / Killing" in that context this is a multi, but real multis are broken down and don't work on individually.
"Villain / Steel can" this is a true multi (Note: Villain is pronounced like Vill-And, Steel is pronounced Still) because it's breaking down the original multiple syllable word into separate syllable rhymes.
There is also simpler multis which just involve groups of words that rhyme with the next line's words.
Example: Sight of death / Flight of Steps
All of these examples are different types of multis which have no specific terms.
Sal: "when you see the sight of men,
You'll be soft like vitamins "
Bob: "That was an insane multi."