Wikipedia says that
an operation $\omega$ is a function of the form $\omega: V \to Y$, where $V \subset X_1 \times\cdots\times X_k$.
But as far as I know, every function's domain is a set, so every function can be seen as an operation where $V \subset X_1$. Thus, all functions are operations. Since all operations are also functions by the definition given, the terms "function" and "operation" are equivalent. What am I missing? Or are they truly equivalent?