A ____ is a hollow metallic tube with a consistent cross-section for transmitting electromagnetic waves by consecutive reflections off the tube's interior walls.

Microwave engineering pertains to the study and design of microwave circuits, components, and systems. Fundamental principles are applied to analysis, design and measurement techniques in this field. The short wavelengths involved distinguish this discipline from electronic engineering. (Source: Wikipedia)
The microwave was invented accidentally in 1945 by a self-taught engineer named Percy Spencer, who was leading a radar project for the defence giant, Raytheon. While testing a new vacuum tube called a magnetron, he discovered that a chocolate bar in his pocket had melted from the heat.