'Freewill Offering' in the Bible

and you make a fire offering to the Lord from the herd or flock—either a burnt offering or a sacrifice, to fulfill a vow, or as a freewill offering, or at your appointed festivals—to produce a pleasing aroma for the Lord,

“You must offer these to the Lord at your appointed times in addition to your vow and freewill offerings, whether burnt, grain, drink, or fellowship offerings.”