Actually there 's no standard, the less turbidity ( or "clarity" ) the clearer the water is, the Secchi disk is nothing but a metallic circle painted in opposing black and white quadrants with a marked rope attached to it.
To measure you drop the gadget and watch it, when you no longer see the 4 quadrants but all blend in a gray shadow you mark with your hand when the event happened, pull out the gadeget and measure the ammount of rope, that tells you the turbidity in inches. "Clear" as a term doesn 't exist, "clear" is less turbid. Terms like Ajj mentioned: clear, stained and muddy do not exist; "muddy" means that the nature ( origin ) of turbidity is formed by suspended particles of soil; "stained" has nothing to do with turbidity, "stained" means that there 's a chemical leach in the water ( like the "black" waters so common in the swamps of the south ) that can be organical or inorganical in origin. '
So there 's a lot of fine print that nobody reads when categorizing turbidity or color, and both along with water temperature and curren, have a deep effect on how bass see your bait and react to it.
You can have "clear" ( less turbidity or low turbidity ) water with a brown "stain" to it. Decaying vegetation leach organic acids ( tannins ) that color and "stain" the water turning it brown, and that brown can be anywhere from a mild yellow to a dark brown. Coffe is a good example to describe stain because you can see it in the flesh, just add water to a cup of coffe and the stain begins to dissapear progessively as you add more water or it can get darker as you remove water from the infusion. Add milk and the tubidity of the coffe cup rises because you are adding particles ( proteins and fat ). It works pretty much the same way in fishing.
That 's why in "clear" water hot or very dark colors, large profile, high noise, strong vibration are not needed, fish can see your bait. Opposed to what happens when the water is "murky" or "muddy".