The Greek noun hagnismos (ἁγνισμός) means purification or cleansing — the act, process, or rite of purifying. It appears once in the New Testament (Acts 21:26), in the context of Paul's participation in a Jewish purification vow. It is the noun form of the verb hagnizo (G48) and derives from hagnos (G53, pure).
The word refers to the ritual process by which someone made themselves ceremonially clean — particularly the purification rites associated with the Nazirite vow or with defilement through contact with a corpse (Numbers 6; 19).
The single New Testament occurrence of hagnismos is historically significant: Paul underwent this Jewish purification rite at the request of James and the Jerusalem elders, to demonstrate to Jewish Christians that he was not teaching Jews to abandon the Law of Moses (Acts 21:20-26). This episode illustrates Paul's principle: "To the Jews I became like a Jew" (1 Corinthians 9:20).
The incident also highlights the transitional nature of the apostolic age: the old covenant rituals were not immediately abolished but gracefully fulfilled as the new covenant was established. The ritual purity of hagnismos pointed forward to Christ — who provides the ultimate purification, cleansing us from sin's defilement with His own blood (Hebrews 9:14; 1 John 1:7). What water and ritual could only symbolize, Christ's blood actually accomplishes.