antibiotic
An antibiotic is a drug that kills bacteria or stops them from multiplying, so that the body's immune system can clear an infection. Think of it as a chemical weapon tuned to attack machinery that bacteria have but your own cells do not — like a key that only fits the locks inside the invader.
Antibiotics work by hitting targets unique to bacteria: building the cell wall, copying DNA, reading the genetic code into protein, or making folate. Because human cells lack a peptidoglycan wall and use different ribosomes, a well-designed antibiotic can be deadly to bacteria yet relatively gentle on the patient. This difference in vulnerability is the whole reason antibiotics are safe enough to swallow.
Historically the word meant a natural substance made by one microbe to kill another (penicillin from a mold), but today it broadly covers any antibacterial drug, including fully synthetic ones. An important honesty check: antibiotics act on bacteria, not viruses, so they do nothing for a cold or flu — and overusing them speeds the rise of resistant strains.
Penicillin, discovered from a Penicillium mold, treats strep throat by blocking bacterial cell-wall synthesis; it is useless against the influenza virus.
A natural-product antibiotic with selective toxicity for bacteria.
Antibiotics are antibacterial; the broader umbrella for drugs against any microbe (bacteria, viruses, fungi, parasites) is antimicrobial. Using the words interchangeably is a common but real error.