Dictionary Definition:

“A principle stating that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint.”

Selected from OxfordDictionaries.com

Functional Definition:

Le Chatelier’s Principle describes the increase-decrease relationship between products and reactants in a chemical reaction. The principal says that if more reactants or products are produced the reaction will move in the opposite direction.  Le Chatelier’s Principle is a description of equilibrium in which every reaction is moving to balance its production of reactants and products.

For example:

Heat +  N₂O₄ (g) <=> 2 NO₂ (g)

Increase in temp the reaction will shift right

Increase in concentration of products it will shift left

Decrease in concentration of products it will shift right