Free available chlorine
Free Available Chlorine (sometimes abbreviated as FAC or FC) - The portion of the total chlorine remaining in chlorinated water that is not combined with ammonia or nitrogen compounds and will react chemically with undesirable or pathogenic organisms. The most active disinfectant in pool/spa water.
To maintain proper sanitation, the FAC should be maintained at between 1.0 and 3.0 ppm. The combined chlorine level should never exceed 0.2 ppm, and ideally should be 0.0 ppm.