This is a term used in cognitive science to describe a theoretical brain cell which fires when you recognize your grandmother.
It is supposed to work like this:
- Your brain gets all sorts of input from your sensory organs. Some neurons are linked directly to this input stream. When a neuron gets the right kind of input, it "fires"; that is, it sends a signal to other neurons linked to it.
- Some neurons in your Brain recieve input from sensory neurons and fire when they detect tiny bits of pattern (edge receptors, phonon receptors, etc).
- Some neurons assemble the tiny bits of pattern into larger patterns. Grey Hair, eyes shaped a particular way, a certain nose, a certain tone to the voice, the smell of a certain perfume, etc.
- Eventually, all the patterns come together and spit out, "Hey it's grandma!", after which the hormonal and emotional responses which you have trained your cerebellum to trigger upon seeing your grandmother get triggered.
The grandmother cell is the theoretical point where input starts becoming output.
Before you get all excited about the concept, please be aware that few scientists actually think that recognition of Grandma is the result of a single neuron firing. Your brain's recognition of Grandma is most likely a complex of such events in several locations.