Unity in a spiritual sense is usually perceived as a return to source, to the one. Union presupposes that there are two (duality) that need to be brought together.
Psychological / emotional perspective: We all attempt to achieve unity from duality – to create a relationship between two parts or opposites. Dreaming of achieving union depicts this relationship. Psychologically, the human being is consistently looking for a partner.
Material aspects: 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 that is for the good of all.