drug
A drug is any chemical you put into a living body that changes how that body works. Think of the body as a vast machine of switches and dials; a drug is a small chemical key that flips some of those switches up or down. Coffee waking you up, an aspirin easing a headache, and an antibiotic killing bacteria are all examples of drugs doing exactly this.
More precisely, a drug is a substance that, when introduced into an organism, alters one or more biological functions through a physical or chemical interaction, usually by binding to a specific molecule. Drugs do not give the body new abilities; they amplify, block, or modulate processes the body already has. A drug can be a tiny synthetic molecule, a hormone, an antibody, or a compound extracted from a plant or microbe.
Importantly, whether a chemical counts as a 'drug' is about how it is used, not about whether it is good or bad. The same substance can be a life-saving medicine at one dose and a poison at another. Water and oxygen are essential to life yet can harm in excess, while a controlled dose of a potent toxin like botulinum can smooth wrinkles or stop muscle spasms. Dose and context decide the outcome.
Aspirin is a drug: a small molecule that inhibits the enzyme cyclooxygenase, reducing the production of pain- and fever-causing prostaglandins.
A familiar drug acting on a single, well-defined target.
In everyday speech 'drug' often means an illegal substance, but in pharmacology it is a neutral technical term covering medicines, vitamins, and toxins alike.