Generalizing the question to functions, note that $\circ$ is just a binary function $\circ(x,y)=z$ but expressed with infixed notation $x\circ y$, in my opinion the common terminology is: given a $n$*-ary function* $f:X_0\times X_1\times...\times X_n\rightarrow Y$
$$f(x_0,x_1,...x_n)=y$$
arguments : $x_0,x_1,...x_n$ are the elements af the domains $X_0, X_1,...X_n$
value : $y$ is the image of the $n$*-uple* $(x_0,x_1,...x_n)$ via the function $f$ and belongs to the codomain $Y$
Note: from wikipedia we get this definition
A value of a function is the result associated to a value of its argument (also called variable of the function)