Final answer:
The projecting flat block under the corona of a Doric cornice is known as a mutule, corresponding to the modillion in other architectural orders.
Step-by-step explanation:
The term you're looking for, a projecting flat block under the corona of a Doric cornice, is known as a mutule. It corresponds to the modillion of other orders, such as the Ionic and Corinthian.
A mutule is found in the Doric order, the earliest of the three Classical orders of architecture, which is known for its plain capital and column shaft that rests directly on the stylobate without a base. The Doric entablature is characterized by a frieze with triglyphs and metopes, and the mutule is an integral part of the entablature's design, located underneath the corona, at the end of each triglyph and metope sequence, creating a distinct visual effect.