The word leaven, as applied to baking, means to lighten dough (i.e., cause it to rise), either chemically, or biologically. There are 3 types of leavening in baking. First, the “unsung” leaveners: These are the leavening agents inherent in the ingredients themselves, and in how they maximize trapped air when they’re combined. Next are the biological leaveners: yeast, both wild (sourdough), and domesticated. Finally, chemical leaveners include baking powder, baking soda, and their chemical siblings.

unsung leaveners
biological leaveners
chemical leaveners