ganglion

noun

1. An encapsulated collection of nerve cell bodies, typically linked by synapses, and often forming a swelling on a nerve fiber.

   “[T]he wonderfully diversified instincts, mental powers, and affections of ants are generally known, yet their cerebral ganglia are not so large as the quarter of a small pin's head.”

2. Any of certain masses of gray matter in the central nervous system, as the basal ganglia.

   “The yawning gap in neuroscientists’ understanding of their topic is in the intermediate scale of the brain’s anatomy. Science has a passable knowledge of how individual nerve cells, known as neurons, work. It also knows which visible lobes and ganglia of the brain do what. But how the neurons are organised in these lobes and ganglia remains obscure.”

3. A centre of intellectual or industrial force, activity, etc.
4. A benign cystic tumour on a tendon sheath or joint capsule.