In the present paper we discuss the relevance for de Sitter fields of the mass and spin interpretation of the parameters appearing in the theory. We show that these apparently conceptual interrogations have important consequences concerning the field theories. Among these, it appeared that several authors were using masses which they thought to be different, but which corresponded to a common unitary irreducible representation (UIR), hence to identical physicals systems. This could actually happen because of the arbitrariness of their mass definition in the de Sitter (dS) space. The profound cause of confusion however is to be found in the lack of connexion between the group theoretical approach on the one hand, and the usual field equation (in local coordinates) approach on the other hand. This connexion will be established in the present paper and by doing so we will get rid of any ambiguity by giving a consistent and univocal definition of a mass term uniquely defined with respect to a specific UIR of the de Sitter group.
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