Saturation
How strong a colour is, from plain grey at zero to its fullest.
See it in FX engine. Under Adjustments, move Saturation all the way down, then all the way up.
In plain words
Saturation is how much colour a colour has. At zero, any colour becomes a grey. At full, it is as strong and pure as it can be. Turning saturation up makes a photo vivid, and turning it down makes it muted. The hue and the lightness stay the same while saturation changes.