Antibiotic-containing paper disks are then applied to the agar and the plate is incubated. If an antibiotic stops the bacteria from growing or kills the bacteria, there will be an area around the disk where the bacteria have not grown enough to be visible. This is called a zone of inhibition.
The presence of an antibiotic-containing paper disk on an agar plate leads to a zone of inhibition around it, indicating that the antibiotic has inhibited or killed the bacteria in that area.