Hebrew: seor (Exodus 12:15, 19; 13:7; Leviticus 2:11), the remnant of dough from the preceding baking which had fermented and become acid.
Hebrew: hamets, properly “ferment.” In Numbers 6:3, “vinegar of wine” is more correctly “fermented wine.”
In Exodus 13:7, the proper rendering would be, “Unfermented things [Hebrew: matstsoth] shall be consumed during the seven days; and there shall not be seen with thee fermented things [hamets], and there shall not be seen with thee leavened mass [seor] in all thy borders.”
The use of leaven was strictly forbidden in all offerings made to the Lord by fire (Leviticus 2:11; 7:12; 8:2; Numbers 6:15).
Its secretly penetrating and diffusive power is referred to in 1 Corinthians 5:6. In this respect it is used to illustrate the growth of the kingdom of heaven both in the individual heart and in the world (Matthew 13:33).
It is a figure, also, of corruptness and of perverseness of heart and life (Matthew 16:6, 11; Mark 8:15; 1 Corinthians 5:7-8).