Figment

When something is a figment of your imagination, it means that you made it up. It's something that might seem real, but is really not.

cardinal

In Catholicism, a cardinal is a high-ranking bishop. In math, you use cardinal numbers to count. A cardinal rule is one that is central and should not be broken.

Okay, that's a lot of definitions. How exactly are they related? In all cases, cardinal means central or essential. It's a cardinal principle that you use it to describe words of behavior like rule or sin. In the Church, cardinals form the central governing body, and in math the cardinal numbers (one, two, three) are the numbers you learn and use first.