Using the logic definition of a structure as a set coupled with finitary functions and relations, what is the definition of the union of two structures $\mathfrak{A}_1 \cup \mathfrak{A}_2$?
I encountered this in a discussion of preservation, where $\mathfrak{A}_1$ is an elementary substructure of $\mathfrak{A}_2$, if that makes any difference.
(I hate to ask such a basic question, but I can't find this anywhere)