A faction is a group of people connected by a shared belief or opinion within a larger group.

In politics, this may refer to any of the several political parties within a country. With any individual party, there may be political factions that hold different beliefs on certain topics.

The word faction is derived from the Latin factiones, a label given to two major Roman political movements, the populares and the optimates. The word factiones was also applied to contemporaneous Roman chariot racing organisations, the Blues, the Greens, the Reds and the Whites.

Faction has also been used to describe a novel based on real historical figures and actual events, woven together with fictional writing. An example of faction is the book According to Queeney by Beryl Bainbridge. This book describes the last few years of Samuel Johnson's life as seen through the eyes of Queeney Thrale, eldest daughter of Henry Thrale and Hester Thrale.

The Factions are the philosophically-derived power groups in the Planescape campaign setting for Dungeons & Dragons game. The philosophical factions controlled Sigil, the City of Doors. Each of the factions is based around one particular belief system; many of the factions' beliefs make them enemies where their other goals and actions might have made them allies. Most factions are organised into covert cells for their own protection. There are fifteen factions in total, per a decree of the Lady of Pain; any additional factions emerging would be subject to her wrath.