A krater (in Greek: κρατήρ, kratēr, from the verb κεράννυμι, keránnymi, "to mix") was a large vase used to mix wine and water in Ancient Greece.

At a Greek symposium, kraters were placed in the center of the room. They were quite large, so they were not easily portable when filled. Thus, the wine-water mixture would be withdrawn from the krater with other vessels. In fact, Homer's Odyssey describes a steward drawing wine from a krater at a banquet and then running to and fro pouring the wine into guests' drinking cups. The modern Greek word now used for undiluted wine, krasi (κρασί), originates from the krasis (κράσις, i.e. mixing) of wine and water in kraters. Kraters were glazed on the interior to make the surface of the clay more suitable for holding water, and possibly for aesthetic reasons, since the interior could easily be seen.

At the beginning of each symposium a symposiarch (συμποσίαρχος), or "lord of the common drink", was elected by the participants. He would then assume control of the wine servants, and thus of the degree of wine dilution and how it changed during the party, and the rate of cup refills. The krater and how it was filled and emptied was thus the centerpiece of the symposiarch's authority. An astute symposiarch should be able to diagnose the degree of inebriation of his fellow symposiasts and make sure that the symposium progressed smoothly and without drunken excess.