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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?
Thermodynamic equilibrium means that:
Rate of forward reaction = Rate of backward reaction
In this case:
$E + Sxrightleftharpoons[k2]{k1} ES at equilibrium: k1.[E][S] iny{ (forward rate)} ormalsize= k2.[ES] iny (backward rate) $
This was the initial assumption in the Michaelis-Menten model.
Later on this was improvised by assuming pseudo-steady state of ES complex. This means that $[ES]$ does not change over time, which is both as a result of its production by the reversible reaction: $E + S leftrightharpoons ES$ and consumption by the irreversible reaction $ ES xrightarrow{k3} E+P$
that is:
$k1[E][S] iny (production) ormalsize=(k2+k3)[ES] iny (consumption)$
