  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.
