What is the fluid-filled space known as in chemical synapses?

At a chemical synapse, the membranes of the pre-and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The presynaptic neuron releases neurotransmitters into the synaptic cleft, which then diffuse across the cleft and bind to receptors on the postsynaptic neuron, leading to a change in its membrane potential. 

Gap junctions are direct connections between the cytoplasm of adjacent cells that allow for the rapid exchange of ions and small molecules. 

Synapse is a junction between two neurons, where information is transmitted. 

Synaptic vesicles are small sacs in the presynaptic neuron that contain neurotransmitters, which are released into the synaptic cleft upon depolarization of the presynaptic membrane.