Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
Voluntary promises which, when once made, were to be kept if the thing vowed was right. They were made under a great variety of circumstances (Gen. 28:18-22; Lev. 7:16; Num. 30:2-13; Dt. 23:18; Judg. 11:30, 39; 1 Sam. 1:11; Jonah 1:16; Acts 18:18, 21:23).