'Free will offering' in the Bible

And thou shalt keep the feast of weeks to the LORD thy God with a tribute of a free-will-offering of thy hand, which thou shalt give to the LORD thy God, according as the LORD thy God hath blessed thee:

That which is uttered by thy lips thou shalt keep and perform; even a free-will-offering, according as thou hast vowed to the LORD thy God, which thou hast promised with thy mouth.