Mr. R.’s World of Math

Jake yelled, “It says Map to the Evil Castle of Math!”
“That’s not real. There’s no evil castle of mathl!” yelled Jenna.
‘Yes there is,” said Joe.
“There is no map!” yelled Jenna, “And there’s no such thing as an evil castle of math either!”
“Wanna’ bet?” said Joe, “My brother told me Mr. Geebrah had him prisoner there for 2 months.”
“You’re so gullible,” said Jenna as she looked at the back of the paper. “Hey, there’s something written on the back of this.”
The students put the papers together again and saw this message:

“Whaterator?” asked Jake who didn’t always pay attention in math class.
“Somethin’ ate-her?” asked Joe who also didn’t always pay attention in math class.
“Don’t you guys know anything! A numerator is part of a fraction,” said Jenna, “The top part! It has nothing to do with eating!”
“Hey, doesn’t that look like Suzy’s handwriting. I think this is her message,” said Jake.
“You’re right! That’s totally Suzy’s handwriting! We have to find the map!” cried Jenna. “Maybe she really is being held in a castle of math!”

If a numerator is the top part of a fraction, what’s the name of the bottom number in a fraction?