Wilson's Dictionary of Bible Types 
1 Corinthians 5:7 (a) Since the word is used to describe the bread in which there is no fermenting yeast, so now it is used to describe the church in which there are no ungodly sinners, none of the Devil's children, but only those who have been washed in the Blood of the Lamb, made pure and beautiful in Christ No evil doctrines would be permitted in such a church.
King James Dictionary 
Unleavened a. unlev'ened. Not leavened not raised by leaven, barm or yeast. Exodus 12 .
Webster's Dictionary 
(a.) Not leavened; containing no leaven; as, unleavened bread.