A pill was originally defined as a small, round, solid pharmaceuticaloral dosage form of medication that was in use before the advent of tablets and capsules. Pills were made by mixing the active ingredients with an excipient such as glucosesyrup in a mortar and pestle to form a paste, then rolling the mass into a long cylindrical shape (called a "pipe"), and dividing it into equal portions, which were then rolled into balls, and often coated with sugar to make them more palatable.
Today, pills include tablets, capsules, and variants thereof like caplets -- essentially anything with medication that can be digested, minus the liquid forms, falls into the pill category.
Some pills are designed to contain sensory and communication elements that collect and wirelessly transmit physiological information after being swallowed.
The oldest known pills were made of the zinc carbonates hydrozincite and smithsonite. The pills were used for sore eyes, and were found aboard a roman ship Relitto del Pozzino which wrecked in 140 BC.