An antibiotic is a substance that can kill or inhibit the growth of microorganisms, such as bacteria, fungus, or parasites. Traditionally, antibiotics refer to substances that are produced by other microorganisms, and do not include synthetic or semisynthetic compounds such as quinolones and amoxicillin, although the term has come to encompass those.

On the other hand, antibacterials specifically act on bacteria only, and encompass not only naturally produced compounds from other microorganisms, but also synthetic compounds, physical agents (such as heat/radiation), and chemical agents (eg. alcohols and halogens).