We don't have synonyms for proprium yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are protium, proprius and proemium.
(n) :
(philosophy) A property that applies to all members of a species and only to them, serving to distinguish the species from other species within the same genus, yet is not part of the true definition or the essence of the species.
(n) :
(philosophy) A property that applies to all members of a species and only to them, serving to distinguish the species from other species within the same genus, yet is not part of the true definition or the essence of the species.