A word for a value between 0 and 1 (inclusive)
I don't think this is a naming issue at all. In the provided example, 'Accuracy' is the correct name for the parameter, as that's what the parameter represents, accuracy. The fact that accuracy should be given as a value in an interval from 0 to 1 should be a property of parameter <i>type</i>. In other words, the parameter should not be a float, but a more constrained type that allows floats only in [0,1].<p>EDIT: Some of you asked what about languages that don't support such more constrained types, so to answer all of you here: different languages have different capabilities, of course, so while some may make what I proposed trivial, in others it would be almost or literally impossible. However, I believe most of the more popular languages support creation of custom data types? 
So the idea (for those languages at least) is quite simple - hold the value as a float, but wrap it in a custom data type that makes sure the value stays within bounds through accessor methods.