The Greek noun ekplerosis is the noun form of ekpleroo (to fulfill completely), meaning a fulfillment or completion. It appears only once in the NT (Acts 21:26), referring to the completion of Paul's Nazirite vow purification — the fulfillment of the required days of purification at the Temple.
The single NT occurrence of ekplerosis is found in one of Paul's most controversial acts. To demonstrate his respect for Torah and his Jewish identity, Paul undertook purification rites with four men who had a vow (Acts 21:23-26). He paid for their offerings and waited for the ekplerosis — the completion — of the purification days. This episode illustrates Paul's principle of becoming "all things to all people" (1 Corinthians 9:22), acting within Jewish practice to reach Jewish people. The tension between his freedom in Christ and his pastoral accommodation is on full display. Fulfillment — of vows, of promises, of the Law — is always in view.