The Tyranid Hivemind is the collective consciousness of the Tyranid race. It knows only one thing: a complete and insatiable hunger.
Official Canon Edit
TTS Canon Edit
The Emperor claims to have made psychic contact with the Hivemind, stating that "it is like talking to a herd of hungry sheep; It is unfulfilling, and makes you look like an idiot." Likewise, the Hivemind connected to the Chaos Gods' warpchat using its synaptic mind to tell them that they were going to eat the galaxy (before the connection dropped). The Hivemind also made contact with latent psyker Bruce Norring, in an image that absolutely horrified him to his core.