Final answer:
A horizontal decorative trim at the top of a building or architecture is called a cornice, found in the entablature section above columns in classical architecture.
Step-by-step explanation:
A horizontal decorative trim located at the top of a building or architectural element is typically referred to as a cornice. In classical architecture, a cornice is the uppermost section of moldings in the entablature, which is supported by columns.
The entablature is divided into three sections: the architrave (the bottom layer directly above the columns), the frieze (the middle, which often contains sculpted or painted decoration), and the cornice, which protrudes from the edge of the building and can serve to protect the structure below from the elements.
Cornices can be found in various architectural orders, such as Doric, Ionic, and Corinthian, each with distinctive features in their entablature designs. For example, in Doric architecture, the frieze may feature alternating triglyphs and metopes, while in the Ionic order, one might find a frieze decorated with continuous sculptural reliefs, and the Corinthian order typically includes more ornate and elaborate cornice designs.