A One Shot is a single, standalone manga that tells a complete story in roughly 15-60 pages. While the majority of the time they consist of a single chapter, One Shots are occasionally several chapters long. One Shots are often released to gauge interest in a full series, in which case they’re also labeled as Pilot Chapters. If popular, these Pilot Chapter/One Shots are serialized into a full series.
Standalone One Shots are sometimes included as specials in a published volume of an unrelated series by the same author, or sometimes are included with a DVD or other merchandise.