A mirror neuron is a neuron which fires both when an animal performs an action and when the animal observes the same action performed by another (especially conspecific) animal.
Thus, the neuron "mirrors" the behavior of another animal, as though the observer were itself performing the action.
These neurons have been observed in primates, including humans, and in some birds.
In humans, they have been found in Broca's area and the inferior parietal cortex of the brain.
Some scientists consider mirror neurons one of the most important findings of neuroscience in the last decade.