Correct spelling, explanation: hilarious is a word of Greek origin, namely the word hilaros, with an English suffix -ious added to make it into an adjective. Besides this change in suffixation, the word did not change much – the structure of the word remains relatively the same, so it’s quite possible that if you asked an ancient Greek what hilarious means, they might have understood.

Definition of hilarious:
adjective
1. very funny
My mom doesn’t like puns but I find them hilarious.
You should see the new comedy, it’s so bad it’s hilarious.

Incorrect spelling, explanation: hillarious is an incorrect spelling of the word hilarious. The proper word comes from the Greek hilaros, which explains why it only needs one l. Otherwise, you may think the word hillarious has something to do with hills or with some Hillary, which is not true. So, try to keep in mind that hilarious only needs one l.