Theory = A set of principles on which to base action, e.g. “A theory of business practices” 2. An idea used to account for a situation, e.g. “My theory is that gremlins mess up my room every night while I’m sleeping 3. Used to describe would is supposed to or could happen, e.g. “In theory, this should work” 4. A scientific system of ideas that explain a physical phenomenon, e.g. “Quantum field theory”