From Fersht, Enzyme Structure and Mechanism p. 87:
The Michaelis-Menten mechanism assumes that the enzyme-substrate complex is in thermodynamic equilibrium with free enzyme and substrate.
In my understanding what this means is that the (E-, S- and ES-concentration dependent) rates of association and dissociation have equated. So we're kind of in this situation:
where "product" would be referring to [ES] and "reactant" to [E] and [S]. Does that make sense?