Before revision: In real transformers, some energy is lost inside to heating of the metallic core of the transformer, to the resistances within the winding conductors, and lost outside to the surrounding environment because of imperfect magnetic coupling between the two coils.
After revision: In real transformers, some energy is lost inside to heating of the metallic core of the transformer, to the AC and DC resistances of the winding conductors, and lost outside to the surrounding environment because of imperfect magnetic coupling between the two coils.