I have heard the terms fat mortar and lean mortar used in the industry. What do these terms mean?

A fat mortar is a mortar that is sticky and adheres to the trowel. This mortar is desirable when laying masonry. A lean mortar, on the other hand, is a mortar that is deficient in its cementitious components. It is usually harsh and difficult to spread and will not stick to the trowel. A lean mortar is usually the result of improper mixing.