If the domain of $B$ is the same as $A$, but you only forget the interpretation of the extra language elements, then $B$ is called a reduct of $A$ to signature $\Sigma'$. But you seem also to be taking a substructure in the smaller language. I have used the term reduct substructure to refer to this relation, but I don't know if there is some other standard terminology. The relation seems to be that $B$ is a substructure of the reduct of $A$ to $\Sigma'$.
Joel David Hamkins
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