Skolem expansion

A Skolem expansion adjoins a Skolem function for every existential first-order formula. Iterating through the successively enlarged languages, and taking their union, gives a language with witness functions even for first-order formulas involving previously added functions. Starting from a language of size $\kappa$, the resulting language has size at most $\kappa +{\aleph }_0$. A hull closed under its functions is a Skolem hull and is elementary in the expanded structure by the Tarski-Vaught test.