Q:

What exactly is a stellar association?

What exactly is a "stellar association" in the strict kinematic sense? 

A:

A stellar association is a loose cluster of stars, that formed at the same time from the same molecular cloud, and so have the same proper motion. Unlike open clusters, they are not gravitationally bound together, so the stars in a stellar association will gradually separate, forming a moving group of stars.
An example is the Scorpius–Centaurus Association

