A hysteresis curve was a mathematical plotting of magnetic field strength and flux density.
As the hull of a Danube-class ship was made up of metallic composites that could interact with magnetic fields, lost ships could be found by using a differential magnetometer to detect fluctuations in the hysteresis curve. (DS9: "Battle Lines")
According to the script, hysteresis was pronounced as "his-teh-REE-ses". [1]