Lenz's Law
Definition
Nature's ‘push back’ rule: induced currents always flow in the direction that opposes the change that created them.
Technical definition
The negative sign in Faraday's law: induced EMF drives a current whose magnetic field opposes the change in flux. A direct consequence of energy conservation — without it, you could get free energy from nothing.
Example
Push a magnet into a coil and the induced current makes the coil's near face repel the magnet — you feel the resistance as you push.
Common confusion
Lenz's law opposes the CHANGE in flux, not the flux itself. If flux is decreasing, the induced field tries to reinforce it. Think ‘oppose the change,’ not ‘oppose the field.’