Union indicates a joining together, and this can be of pairs or of multiples. Union in pairs suggests the reconciliation of opposites and the added energy this brings.
A union, in the sense of a trade union, suggests collective action which is for the good of all.
We all attempt to achieve unity from duality - to crcate a relation- ship between two parts or opposites.
Dreaming of achieving union depicts this relationship. Psychologically, the human being is consistently looking for a partner.
Unity in a spiritual sense is usually perceived as a return to Source.
[1]United with others in a common cause.
[2]Unity of purpose
[3]