Final answer:
An epistyle, commonly known as an architrave, is the correct term for a beam that spans the space between two piers or columns, or between a pier or column and a wall, serving as a key structural element in classical architecture.
Step-by-step explanation:
A beam that spans the space between two piers or columns, or between a pier or column and a wall is called an epistyle, or more commonly referred to as an architrave in classical architecture. This component serves as a key structural and aesthetic element, resting directly above the columns or piers. In Romanesque architecture, which often features the alternation of piers and columns, such beams would contribute to the support of structures like naves in churches, where wooden roofs with trusses or timber beams might be found above. These beams are important for distributing the weight of the roof or upper levels of a building.
Epistyles are distinct from other architectural elements such as a cornice, which is a decorative moulding along the top of a building or wall, and a pediment, which is a triangular upper part of a classical building, typically surmounting a portico of columns. While architraves serve a clear structural purpose, cornices and pediments serve more to adorn and cap-off the structures on which they rest. An epistyle holds a practical role akin to the lintel, which supports the weight above doorways and windows.