A single intelligence inhabiting multiple instances.
A Gravemind is the apex of the Flood's coordinating intelligences. It is a monument of ancient sins; a self-aware compound mind that transcends individual hosts and lesser key minds; a formless intellect who knows all that the Flood have ever possessed.
The Gravemind brings its horrific form of peace through unification, salvation through assimilation. Its only goal is the consumption of every thinking creature in the galaxy. The Gravemind's most recent physical avatar was destroyed when the Master Chief fired the Ark's incomplete Halo, but it is only a matter of time before it rises again.