We all know the word's meaning as an adjective and a verb (if you don't, head on over to Merriam-Webster; they'll help you out!), but today I learned it can also be used as a noun.
Shocking, I know.
Again, a la Merriam-Webster: an agreement or covenant between two or more parties
Googling "compact agreement" seems to imply that a compact is usually used in a legal or official setting. I'm not sure how a contract could get more official, but if it was going to, it'd become a compact.
P.S. The quote in this post is by William Lloyd Garrison, an abolitionist from the 1800s who thought the Constitution was sadly soft on the issue of owning slaves. Check out PBS's Garrison biography for more information.