A graph or network is said to be fully connected if there is a link between every possible pair of nodes. The term is also often applied to adjacent layers in a neural network in this case it means that each node in one layer is connected to every node in the next layer, but not that the nodes within a layer itself are connected.
Used in glossary entries: neural network
Boltzmann Machine – a fully connected network
Restricted Boltzmann Machine – fully connected between layers, but not within layers