Short answers, pulled from the story.
A one-shot comic is a work composed of a single standalone issue or chapter, with no continuation. It contrasts with limited series and ongoing series, which span multiple issues. One-shots appear in print comic books, comic magazines, anthologies, and online.
In Japan, a one-shot manga is called yomikiri, a term that implies the work is presented in its entirety without any continuation. Most modern yomikiri carry independent settings, characters, and storylines rather than sharing them with existing works.
Dragon Ball, Fist of the North Star, Sailor Moon, Naruto, Bleach, One Piece, Berserk, Kinnikuman, Attack on Titan, and Death Note all began as one-shot manga before becoming full serialized series.
In the United States, one-shots are usually labeled with a "#1" despite having no following issues and are sometimes subtitled as "specials." Publishers use them for characters popular enough to publish regularly but not financially lucrative enough for an ongoing or limited series, often releasing them annually or quarterly.
Rising Stars of Manga was an annual competition for original English-language one-shot manga. Many of the entries went on to become full-length manga series.
One-shot comics date back to the early 19th century, first appearing in newspapers. Western prototype comic works took the form of pamphlets, giveaways, and Sunday newspaper comic sections during the 1800s.