Definition.
Constellation is a term used to describe the state when a complex is active and blending with the ego. When we say we are ‘triggered,’ this almost certainly reflects the constellation of a complex.
Constellations happen quickly and in response to some sort of cue: the arrival of emotionally charged situation, a reminder of a past trauma, a challenge to our autonomy in some area, etc. Once the affect-energy of the complex is expended, the constellation ends, and the complex returns to the unconscious. Constellations may be followed by bafflement: Where on earth did that come from? That wasn’t like me, at all.
An illustration may be helpful. Imagine that you are driving a large van. Your unconstellated complexes, or subpersonalities, sit in the rows of seats behind you. When a complex becomes constellated, it may either 1) take the seat beside you, perhaps advising or nagging you, and thus impacting your driving decisions, or 2) bump you out of the driver’s seat entirely, essentially taking control of the van.