a piece of sour dough, used to ferment and render light a much larger quantity of dough or paste.

Leaven was strictly forbidden by the law of Moses during the seven days of the passover; and the Jews, in obedience to this law, carefully purified their houses from all leaven, as soon as the vigil of the feast began. Nothing of honey or leaven was to have place in any thing presented to the Lord, upon his altar, during this solemnity. If, during the feast, the least particle of leaven was found in their houses, they imagined that the whole was polluted, for a little leaven leaveneth the whole lump. Leaven, in its figurative sense, signifies the bad passions of envy, and malice, and rancour, which sour the temper, and extend their fermenting influence over the social affections; whereas unleavened bread implies sincerity and truth. It is frequently used for any kind of moral contagion.