a chiasm (sp) is a crossing over. EG.
your pituitary is located underneath your supra-optic-chiasm (eye crossing over).
that is why when you have pititutary enlargement it is often accompanied by visual defects/loss. b/c the enlarged pit. is pressing against the optic nerves.
a synapse can also mean a crossing over area. as in neurons => denrite goes to cell body down the axon across the synapse to an effector/another neuron.