Literally, "conjunction," used in alchemy to refer to chemical combinations;
psychologically, it points to the union of opposites and the birth of new possibilities.

The coniunctio is an a priori image that
occupies a prominent place in the history of
man's mental development. If we trace this
idea back we find it has two sources in
alchemy, one Christian, the other pagan.
The Christian source is unmistakably the
doctrine of Christ and the Church, sponsus
and sponsa, where Christ takes the role of
Sol and the Church that of Luna. The
pagan source is on the one hand the
hieros-gamos, on the other the marital
union of the mystic with God.
C. G. Jung, "The Psychology of the Transference," CW 16, par. 355