Legal Words:

In an effort for us all to seem more smart... (more smart.. he he)
I will post legal terms that you can use in your everyday life.

si·ne qua non : (sĭn'ĭ kwä nŏn', nōn', sī'nĭ, kwā) n

[Latin, Without which not.] A description of a requisite or condition that is indispensable.
In the law of torts, a causal connection exists between a particular act and an injury when the injury would not have arisen but for the act. This is known as the but for rule or sine qua non rule.

example of everyday use
“The perfect music is the sine qua non of the carefully planned modern wedding”