Sourdough which is kept over from one baking to another, in order to raise the new dough. Leaven was forbidden in the Hebrews during the seven days of the Passover, in memory of what their ancestors did when they went out of Egypt, they being then obliged to carry unleavened meal with them, and to make bread in haste, the Egyptians pressing them to be gone, Exodus 12:15,19 . They were very careful in cleansing their houses from it before this feast began, 1 Corinthians 5:6 . God forbade either leaven or honey to be offered to him in his temple, Leviticus 2:11 . The pervading and transforming effect of leaven is used in illustration of the like influence on society, exerted by the purifying principles of the gospel, or by false doctrines and corrupt men, Matthew 12:23 16:6-12 1 Corinthians 5:6-8 .
Rand, W. W. Entry for 'Leaven'. American Tract Society Bible Dictionary. https://www.studylight.org/dictionaries/ats/l/leaven.html. 1859.