Uniform Definability and Undecidability in Classes of Structures

We present a concept of uniform encodability of theories and develop tools related to this concept. As an application we obtain general undecidability results which are uniform for large families of structures. In the way, we define uniformly in the characteristic the equivalence relation "$x\sim y$ if and only if $x$ is an iterate of $y$ throuh the Frobenius map, or vice versa" in large classes of function fields and in polynomial rings.