The point where the demand for chlorine has been fully satisfied in terms of chlorine addition to the water.

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This is about breakpoint chlorination—the moment the water’s chlorine demand has been met. As chlorine is added, it reacts with substances in the water (organic matter, ammonia, reduced compounds) and is consumed, so the free chlorine residual stays low. When all that demand is satisfied, any additional chlorine remains as free chlorine, and the residual begins to rise. That transition point is the breakpoint, the threshold where disinfection becomes effective with a measurable free chlorine residual. The other terms aren’t used for this concept: breakthrough refers to contaminants slipping through a barrier, contact point isn’t the standard term for this chlorine dosing event, and saturation point isn’t the correct label for breakpoint chlorination.