Q: In mathematics, what is a quarter of a circle called? (an area bounded by two radii at right angles to each other)
A: quadrant

Q: What form of communication is an epistle?
A: letter

Q: Which thermodynamic concept, defined as the unavailability of a system's energy to do work, or more crudely, the measure of disorder within a system, was introduced by the German physicist Rudolf Clausius?
A:
entropy