Chemist Henry Louis Le Chatelier (1850-1936) was the first to notice that different phenomena thermochemistry obeying a principle can be stated as: "If you tend to change the terms thermodynamic (pressure, temperature, concentration) from one system to the balance, it reacts to oppose, in part, to changes that it requires, so as to obtain a new balance. " 

The German and the Nobel Prize for Physics Karl Ferdinand Braun (1850-1918) had also discovered and formulated this principle.
Balance is a state where the speed of transformation of substrate into a product, in a reversible reaction, is equal to the speed of processing to the substrate. Thus, the apparent speed of a reaction is reversible at equilibrium is zero.