A regular language over an alphabet $\Sigma$ is a subset of the set of all words over $\Sigma$ that can be accepted by some finite automaton. A regular language identifies a certain property of strings of $\Sigma^*$.
One can then define a regular predicate over $\Sigma^*$
as an n-ary relation on $\Sigma^*$ such that by suitably coding n-tuples of words as single words (see for example Blumensath and Gradel) one obtains a regular language (over a new alphabet). If $R(x,y)$ is a regular binary predicate, what can be the status of the unary predicate $\exists x.R(x,y)$? (again regular, decidable, r.e., or what else?)