Soda bread is a quick bread leavened with baking soda instead of yeast - the soda reacts with an acidic ingredient (traditionally buttermilk) to release carbon dioxide and make the dough rise, so there is no proofing time. It is most closely associated with Irish cooking, particularly as part of a traditional Irish breakfast.
The traditional version uses just flour, baking soda, buttermilk, and salt, though whole wheat flour, raisins, or caraway seeds are common additions. It has a dense but tender crumb and a slightly tangy flavor from the buttermilk, with a firm, chewy crust.
Similar quick breads leavened with baking soda instead of yeast turn up elsewhere too, such as Scottish bannock. Soda bread is best eaten the day it is baked, since without yeast's structure it dries out and turns crumbly faster than a yeasted loaf.