Broadly speaking anything you add to a bread recipe that improves the bread could be called a "bread improver". The term is most commonly used to describe a range of natural or chemical additives that speed up the process of dough development.
Bread improver is especially important in large scale bread production but is also often found in bread machine pre-mixes and other recipes baked in the home kitchen. Bread improver may also be known by other names such as yeast improver, yeast food, dough enhancer, dough conditioner or bread enhancer.
For the home baker looking for a way to make even better bread at home there are plenty of choices. You can even find improvers made from entirely natural ingredients that are good for you as well as your bread.