An antibiotic generally refers to a chemical that can be used on or inside a patient (humans, pets, livestock, etc.) to inhibit the growth of bacteria (bacteriostatic) or kill bacteria (bactericidal).
Antibiotics are the substances which are derived from one microorganism in order to kill another microorganism. Antibiotics are effective against bacterial, fungal and parasitic infections.
Most antimicrobial drugs express their effect either through being static (e.g. bacteriostatic) or cidal (bactericidal). Static drugs stop the organism from growing but do not necessarily kill it; cidal drugs generally kill the microorganism at concentrations that can be achieved clinically.
An antimicrobial agent is defined as a natural or synthetic substance that kills or inhibits the growth of microorganisms such as bacteria, fungi and algae.