Webster's Dictionary 
(1): ( n.) A sacrifice, or ceremony, by which cities, fields, armies, or people, defiled by crimes, pestilence, or other cause of uncleanness, were purified.
(2): ( n.) The act of lustrating or purifying.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
a formal and public application of water in token of consecration or expiation. Such acts were prevalent not only among heathen nations, more especially those of the southern climates, such as the Indians, Egyptians, Greeks, and Romans (compare Wetstein. Nov. Test. Evang. Matthew 3:6), but also among the Jews (see H Ä ner, De lustratione Hebraeorum, Wittenb. 1733). With these latter they were preparations for divine services of a different nature, and even for private prayer (Judith 12). They formed a part of the offering-service, and more especially of the sin-offering (Leviticus xvi); and for that reason the prayer-houses ( Προσευχαί ) were usually established in the vicinity of running waters (compare Kuinol, ad Acts 16:13). Josephts ( Ant. 18, 1:5) gives an account of the manifold lustrations of the Essenes. In the language of the prophets, cleansing with water is used as an emblem of the purification of the heart, which in the Messianic age is to glorify the soul in her innermost recesses, and embrace the whole of the theocratic nation ( Ezekiel 36:25 sq.; Zechariah 13:1). Such declarations gave rise to or nourished the expectation that the advent of the Messiah would manifest itself by a preparatory lustration, by which Elijah or some other great prophet would pave the way for him. This supposition lies evidently at the bottom of the questions which the Jews put to John the Baptist ( John 1:25; compare Matthew and Luke 3:7), whether he was the Messiah, or Elijah, or some other prophet? and if not, why he undertook to baptize? (compare Schneckenberger, Ueber das Alter der Judischen Proselytentaufe, § 41 sq.). Thus we can completely clear up the historical derivation of the rite, as used by John and Christ, from the general and natural symbol of baptism, from the Jewish custom in particular, and from the expectation of a Messianic consecration. (See Baptism).
Among the ancient Greeks, and more particularly the Romans, lustrations were of most solemn import. Those of which we possess direct knowledge are always connected with sacrifices and other religious rites, and consisted in the sprinkling of water by means of a branch of laurel or olive, and at Rome sometimes by means of the aspergillum, and in the burning of certain materials, the smoke of which was thought to have a purifying effect. Whenever sacrifices were offered, it seems to have been customary to carry them around the person or thing to be purified. Lustrations were made in ancient Greece, and probably at Rome also, by private individuals when they had polluted themselves by any criminal action. Whole cities and states also sometimes underwent purifications to expiate the crime or crimes committed by a member of the community. The most celebrated purification of this kind was that of Athens, performed by Epimenides of Crete, after the Cylonian massacre. Purification also took place when a sacred spot had been unhallowed by profane use, as by burying dead bodies in it, as was the case with the island of Delos. (See Ablution).
The Romans performed lustrations on many occasions on which the Greeks did not think of them, and the object of most Roman lustrations was not to atone for the commission of crime, but to obtain the blessing of the gods upon the persons or things which were lustrated. Thus fields were purified after the business of sowing was over, and before the sickle was put to the corn. Sheep were purified every year at the festival of the Palilia. All Roman armies before they took the field were lustrated, and, as the solemnity was probably always connected with a review of the troops, the word lustratio is always used in the sense of the modern review. The establishment of a new colony was always preceded by a lustratio with solemn sacrifices. The city of Rome itself, as well as other towns within its dominion, always underwent a lustratio after they had been visited by some great calamity, such as civil bloodshed, awful prodigies, and the like. A regular and general lustratio of the whole Roman people took place after the completion of every lustrum, when the censor had finished his census, and before he laid down his office. This lustratio (also called lustrum) was conducted by one of the censors, and held with sacrifices called Suovetaurilia, because the sacrifices consisted of a pig (or ram), a sheep, and an ox. It took place in the Campus Martius, where the people assembled for the purpose. The sacrifices were carried three times around the assembled multitude. See Smith, Dict. of Class. Antiquities, s.v. Lustratio.
Something of the nature of lustration prevails in the use of "holy water" (q.v.) by the Roman Catholics.