abbreviation for Jackson turbidity unit, a unit formerly used in measuring water quality. The turbidity of water was measured by lighting a candle under a tall glass tube and filling the tube with the water sample until a observer looking down the tube could no longer see the candle flame through the water. The height of the water column determines the turbidity in Jackson turbidity units, using a table constructed for this purpose. Turbidity is usually measured now in a different unit, the NTU, using an instrument called a nephelometer.