Learning algorithm for a neural network. Named in honour of the neuropsychologist Hebb:
    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 changes take place in one or both cells such that A's efficiency as one of the cells firing B, is increased.

- Hebb, The Organization of Behaviour

What this means is the weights between synapses increase if the two neurons are activated simultaneously and it is reduced is they are activated asynchronously.