From H7604 (sha'ar, to remain). Leaven β fermented dough that causes bread to rise. Strictly prohibited during Passover and the Feast of Unleavened Bread.
S'or carries enormous symbolic weight. The removal of leaven during Passover symbolized urgency and purity. Leaven represents corruption β the 'puffing up' of what should remain simple. Jesus picks up this imagery: 'Beware of the leaven of the Pharisees.' Paul: 'A little leaven leaveneth the whole lump.'