'Vow' in the Bible

And Paul after this tarried there yet a good while, and then took his leave of the brethren, and sailed thence into Syria, and with him Priscilla and Aquila; having shorn his head in Cenchrea: for he had a vow.

Do therefore this that we say to thee: We have four men which have a vow on them;

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.

and they incur disapproval for having broken their original vow.

You adulteresses [disloyal sinners—flirting with the world and breaking your vow to God]! Do you not know that being the world’s friend [that is, loving the things of the world] is being God’s enemy? So whoever chooses to be a friend of the world makes himself an enemy of God.