A capsule was a pill in the form of a small rounded gelatinous container with medicine inside.
In 2269, while Nurse Chapel and Harry Mudd were discussing the love potion and using the love potion crystal on Spock, Chapel thought that she would just break the capsule and rub it on her skin, and then touch Spock, so she broke the capsule between her hands. (TAS: "Mudd's Passion")