They are not synonyms - psychoanalysis is a specific form of psychodynamic therapy (and even then there are variations - from traditional Freudian analysis to object relations, Kleinian, Feminist etc flavours). Psychodynamic therapy (therapies really) is a broader and looser group of approaches, all of which have their origins in psychoanalysis but some have moved far away from those origins (eg embodied relational therapy would be one modern example of a psychodynamic theory approach that would not be classed as psychoanalysis).