What is the difference between leavened and unleavened bread?

If bread is made without a rising agent it is called unleavened, or flatbread; the ingredients for unleavened bread can be reduced to flour, water, and olive oil.  If a rising agent, like yeast or baking soda, is added to the recipe, the bread will be undergo a leavening, or rising, action, producing leavened bread. Leavened bread is pervasive in Western countries while unleavened bread is fairly uncommon, at least outside of a religious context.