'Vow' in the Bible

Paul stayed for a while longer, and then told the brothers and sisters goodbye and sailed for Syria; and he was accompanied by Priscilla and Aquila. At Cenchrea [the southeastern port of Corinth] he had his hair cut, because he was keeping a [Nazirite] vow [of abstention].

Therefore do just what we tell you. We have four men who have taken a vow;

Then Paul took the [four] men, and the next day he purified himself along with them [by submitting to the ritual]. He went into the temple to give notice of [the time] when the days of purification [ending each vow] would be fulfilled and the usual offering could be presented on behalf of each one.