These muffins are an example of quick breads.
quick bread definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
any bread, as muffins or corn bread, leavened with baking powder, soda, etc. so that it may be baked as soon as the batter or dough is mixed
quick bread definition by American Heritage Dictionary
A bread made with a leavening agent, such as baking powder, that expands during baking and requires no leavening period beforehand.