Synapse is the Tyranid Psychic Power which unites all of the Tyranids into the Hive Mind. It is projected by Synapse Creatures which are usually leaders such as Hive Tyrants, Tyranid Warriors and Broodlord or specialist psykers like Zoanthropes. These creatures have greatly increased synapse networks within their cortex and these act as psychic conduits for the Hive Mind to order nearby troops. Without the Synapse guiding them, most Tyranids will revert to a feral state, although some, such as Genestealers with their Brood Telepathy, can perform just as well without Synapse as they do with it.1-p28
In 2nd Edition this power was called Hive Mind.2-p18