sine qua non (n.)
- catch
- clause
- climax
- condition
- core
- cornerstone
- crisis
- crux
- essence
- essential
- fundamental
- gist
- given
- grounds
- heart
- issue
- joker
- kernel
- keystone
- kicker
- landmark
- meat
- milestone
- must
- necessity
- nub
- obligation
- parameter
- pith
- pivot
- prerequisite
- provision
- provisions
- proviso
- requirement
- requisite
- specification
- stipulation
- string
- substance
- terms
- ultimatum