Mechanical energy
Also called Total mechanical energy
The sum of a system's kinetic and potential energies. It stays constant only when the work done on the system is zero and there are no nonconservative interactions inside the system.
Add a system's kinetic and potential energies together and you have its mechanical energy. That is the CED's definition, and it is only a definition: whether the sum holds still is a separate question with a precise answer.
The condition comes from Topic 3.4. If the work done on a selected system is zero and there are no nonconservative interactions within the system, the total mechanical energy of the system is constant. Both halves have to hold. An external hand pushing on the system breaks the first one. Friction or drag acting inside it breaks the second.
When mechanical energy is not conserved, nothing is actually lost. The CED expects students to know that mechanical energy can be dissipated as thermal energy or sound by nonconservative forces, and states separately that energy is conserved in all interactions. What changed is the accounting category, not the total.
The choice of system decides the verdict. Put a sliding block on its own inside the boundary and friction is an external force doing negative work on it. Widen the boundary to include the surface and the thermal energy that ends up in it, and nothing crosses.
Setting up and solving those balances is the job of the conservation of energy guide.