Le Chatelier's Principle
Definition
If you disturb a system at equilibrium, it shifts to fight the disturbance: add reactant and it makes more product; raise temperature and it shifts to absorb the heat.
Technical definition
Le Chatelier's principle states that when a stress such as a concentration, pressure, or temperature change is applied to a system at equilibrium, the system shifts to partially counteract it. Only temperature changes alter K itself, while concentration and pressure shifts change the reaction quotient Q.
Example
In the Haber process for ammonia, raising pressure shifts equilibrium toward ammonia because that side has fewer gas molecules.
Common confusion
Adding a catalyst does NOT shift equilibrium. It speeds up both directions equally, so you reach the same equilibrium faster, not a different one.