Synaptic is the adjective for a synapse — the tiny junction where two nerve cells pass a message across a gap. It almost never stands alone; it sits in front of another word, in phrases scientists use again and again: 'synaptic gap', 'synaptic connections', 'synaptic activity', 'synaptic pruning' (when a growing brain quietly gets rid of connections it does not use). You will meet it in science writing, in psychology classes and in articles about how learning changes the brain, and hardly ever in conversation. Say sin-AP-tic, with the stress in the middle — which is different from the noun SIN-aps.