Rashi on Leviticus 22:18/רש"י על ויקרא כ״ב:י״ח

1נדריהם THEIR VOWS — [An animal is termed נדר when an obligation is entered into by the words:] Behold, I take it upon myself [to bring a burnt-offering].

2נדבותם THEIR FREE-WILL OFFERINGS — [it is a נדבה when one states:] Behold, this animal [shall be a burnt-offering] (Megillah 8a).