Wiki · Reference entry
Readiness
Whether a system is actually fit to act, as opposed to whether it looks finished.
The distinction sounds pedantic until you have shipped something. Looking finished is a property of what you can see: the tests, the demo, the score. Being ready is a property of the world, including the requirements nobody wrote down. The two are frequently far apart, and the distance is not visible from inside the project.
Why it matters
The whole argument on this site is that these two are different numbers.