Easton's Bible Dictionary
Heb. seor (Exodus 12:15,19; 13:7; Leviticus 2:11 ), the remnant of dough from the preceding baking which had fermented and become acid.
Heb. 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 [Heb. 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 chemical definition of ferment or yeast is "a substance in a state of putrefaction, the atoms of which are in a continual motion."
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 ).
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain.
Easton, Matthew George. Entry for 'Leaven'. Easton's Bible Dictionary. http://www.studylight.org/dictionaries/ebd/view.cgi?n=2259. 1897.