Proofing (also called proving or more rarely blooming), as the term is used by professional bakers, is the final dough-rise step before baking, and refers to a specific rest period within the more generalized process known as fermentation. Fermentation is a step in creating yeast breads and baked goods where the yeast is allowed to leaven the dough.
Fermentation rest periods are not often explicitly named, and normally appear in recipes as "Allow dough to rise."
Proofing yeast may refer to the process of first dissolving yeast in warm water, a needed hydration step when using active dry yeast. In one variant of the sponge technique known as poolish, proofing the yeast is the process step prior to feeding the yeast any carbohydrate. Proofing, as used in some cookbooks intended for home use, may refer to testing the viability of yeast
by feeding it a carbohydrate
Also see Dough Retarder