Definition:

The cornice is the uppermost section of moldings along the top of a wall or just below a roof.

In Classical architecture, the cornice is the upper portion of the entablature, located above the architrave and the frieze.

Don't worry if this is too difficult to comprehend. One dictionary describes it this way:

"cornice 1. Any molded projection which crowns or finishes the part to which it is affixed. 2. The third or uppermost division of an entablature, resting on the frieze. 3. An ornamental molding, usually of wood or plaster, running round the walls of a room just below the ceiling; a crown molding; the molding forming the top member of a door or window frame. 4. The exterior trim of a structure at the meeting of the roof and wall; usually consists of bed molding, soffit, fascia, and crown molding."—Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 131

A way to remember this architectural detail is to know where the word comes from—the etymology or origin of the word. Cornice is, indeed, Classical because it comes from the Latin word coronis, meaning curved line, and the Greek word for a curved object, koronis—the same Greek word that gives us our word crown.