ECVs are measures of a drug MIC distribution that separate bacterial into a wild type and non-wild type (acquired or mutational resistance to the drug) populations. A bacterium with a drug MIC greater than the ECV is likely to have an acquired form of resistance. The ECV is NOT the same as a susceptibility breakpoint, and is difficult to apply clinical decisions regarding antibiotic therapy. The ECV will however be informative when limited data is available regarding the correlation between MIC and clinical responses and when robust in vivo and in vitro data about a drug pharmacokinetics and pharmacodynamics is lacking. You can read a more comprehensive definition and a few examples here.