Quoting the Proofwiki definition of mononomials:
A mononomial in the indexed set $\displaystyle {\{X_j:j\,{\in}\,J}\}$ is a possibly infinite product
$$\displaystyle \prod_{j \mathop \in J} X_j^{k_j}$$
with integer exponents $k_jā„0$ such that $k_j=0$ for all but finitely many $j$.
How to understand this definition? It talks about a product of elements $X_j$ - how is this operation defined?