Final answer:
The term for a flattened edge of a solid member on a plane at a 45-degree angle to adjoining planes is called a chamfer.
Step-by-step explanation:
The flattening of a longitudinal edge of a solid member on a plane that lies at an angle of 45 degrees to the adjoining planes is called a chamfer. This is often done to remove sharp edges for safety, aesthetic purposes, or to prepare an edge for a more complex machining process. The term chamfer applies in various contexts where an edge is beveled at a specific angle, not strictly horizontal or vertical, but, in this case, at a 45-degree angle.