WordNet records no synonym and no antonym for introjection. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(psychoanalysis) the internalization of the parent figures and their values; leads to the formation of the superego
(psychology) unconscious internalization of aspects of the world (especially aspects of persons) within the self in such a way that the internalized representation takes over the psychological functions of the external objects
Synonyms, antonyms and the broader and narrower relations for introjection come from Princeton University’s WordNet. Full sources and licences.