Let $X$ denote a model of an algebraic theory $T$. What do we call the functions $f : X^n \rightarrow X$ that are definable by some expression in the language of $T$?
e.g. If $S_3$ is the symmetric group, then the function $f : S_3 \rightarrow S_3$ defined by $f(x,y) = xyyx$ is definable by the expression $xyyx$, and also by the expression $xyyyy^{-1}x$.