Bread baked from unfermented dough, or dough without yeast or "leaven" . Unleavened bread was the flat bread used in the PASSOVER celebration and the priestly rituals . The tradition of eating unleavened bread goes back to the time of the Exodus, when the Hebrews left Egypt in such haste that they had no time to bake their bread . Leaven was produced by the souring of bread dough. Its exclusion from ceremonial breads probably symbolized purity.

(from Nelson's Illustrated Bible Dictionary)

(Copyright (C) 1986, Thomas Nelson Publishers)