A stellar association is a loose group of stars of similar spectral class and of recent origin, thought to be the result of a recent molecular cloud's star formation, something like an open cluster only looser. They generally form a moving group, a group of stars near each other in position and peculiar velocity, which can be identified through astrometry over time, which is a primary means of identifying stellar associations. The associations that can be identified are generally in nearby portions of the Milky Way, are somewhat young (before the stars have scattered much), and the peculiar velocity is a necessary identification factor because they can overlap in the celestial sphere and can reside in the same volume of space. At their distance, they can cover hundreds of square degrees.
It is thought that most stars are born in stellar associations. They can be typed by their stellar class (such as OB associations, with both O-type stars and B-type stars), other classes being R associations (if they illuminate a reflection nebula) and T associations (with T-Tauri star).