Just adding it because I think it's a cool use of the word.
When reaction wheels are operating at maximum speed and can no longer exchange momentum with the spacecraft in both directions, they are considered 'saturated'.
'Saturated' is used in control systems engineering for any value (sensor input, drive output etc.) that has reached maximum or minimum value and can't be further increased/decreased. It's the same sense of the word as used in the HSV colour space in computer graphics.