Energy
Energy is a scalar property of a system, measured in joules, that can be converted between types or transferred across the system boundary but is never created or destroyed. There is no single equation for energy, only one per type.
Energy has no defining formula in AP Physics, and looking for one is the mistake. What the course gives you is a set of accounts, one per type, plus a rule that the total never changes.
It belongs to a system, not to a substance. EK 3.4.A.1 is blunt about this: a system composed of only a single object can only have kinetic energy. Potential energy needs at least two objects that interact through a conservative force, per EK 3.3.A.1, so a lone falling ball has no gravitational potential energy until you put Earth in the system with it. Change the system and you change the answer.
Conserved is not the same as constant. EK 3.4.C.1 says energy is conserved in all interactions, full stop. Whether the total inside your chosen system holds still is a separate question: EK 3.4.C.2 makes it constant when the work done on the system is zero and there are no nonconservative interactions inside it, while EK 3.4.C.3 says nonzero work transfers energy across the boundary.
Units and symbol. The joule, J, equal to a newton meter. The AP equation sheet's symbol list gives = energy, and the sheet prints .
The types the course counts separately are kinetic energy and potential energy in its gravitational and elastic forms, whose sum for a system is its mechanical energy, plus the thermal energy or sound that nonconservative forces dissipate. The solving routine is in conservation of energy.