What is OSU unstable rate?
According to the osu! wiki: Unstable rate represents the consistency of the timing of the hits, where lower numbers are better (top players often score below 100).
How is OSU calculated?
The formula is essentially the standard deviation of your hit errors (in milliseconds) multiplied by 10.
What does ur stand for in OSU?
Unstable Rate. It's roughly 10 * the standard deviation of your hit timing from perfect in milliseconds.