A bread made without leaven (yeast). In ancient times, bread makers used a piece of dough left from a previous bake. It had fermented and developed a certain acid content. This was the yeast that caused bread to rise.
By God's command, bread for the Jewish Passover must be unleavened. This applies to most other religious observances, too (Exodus 12:15–20; 23:15). The people could use leavened bread for spiritual purposes only in certain cases (Leviticus 7:13; 23:17). This was due to leaven's symbol of evil; fermentation implied decay.
The New Testament speaks of leaven negatively, except for Jesus' teaching on the kingdom of God (Matthew 13:33).
Jesus warned of the leaven of the Pharisees and Sadducees (Matthew 16:6)
Paul urged believers to guard against unhealthy values. They can spread like yeast in the dough (1 Corinthians 5:6–8).