A learning rule, which is based on physiological observations of brains, neurons, and their synapses and axons.

From Donald Hebb's book The Organization of Behaviour:

"When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased."

This forms the basis of associative learning, and is often shortened to: "If it fires together, it wires together."

• Neuroscientists discover new learning rule for pattern completion
  Really a new experiment that tends to contradict an STDP (Spike-Timing-Dependent-Plasticity) association rule that has long been held to be almost axiomatic amongst STDP adherents.