Your dough's ability to stretch without tearing -- the opposite of elasticity.
Extensibility comes from the gliadin proteins and from gluten bonds being broken down by protease enzymes and bacterial proteolysis during fermentation. A more extensible dough is easier for yeast to inflate, resulting in a lighter, airier crumb. But too much extensibility (overfermentation) means the dough can't hold its shape at all. The balance between elasticity and extensibility is the core tension in sourdough baking.
