(from Latin lustratio, “purification by sacrifice”), any of various processes in ancient Greece and Rome whereby individuals or communities rid themselves of ceremonial impurity (e.g., bloodguilt, pollution incurred by contact with childbirth or with a corpse) or simply of the profane or ordinary state, which made it dangerous to come into contact with sacred rites or objects. The methods varied from sprinkling with or washing in water, through rubbing with various substances, such as blood or clay, to complicated ceremonies, some of which involved confession of sins. Fumigation was also used.

When a community was to be purified, either from collective guilt or from the accumulated ill luck and ill-doing of a period of time, different processes were used from culture to culture. The usual Greek method, for instance, seems to have been to lead through the village certain persons or animals capable of absorbing the pollution and then to lead them out of the city. In Rome, purifying materials were led or carried around the person or community in question. Many noteworthy public rites were of this kind, such as the Lupercalia (around the Palatine hill) and the amburbium (“around the city”).