Final answer:
The hedgehog maintains a stable internal environment through homeostasis, which is a biological process of constantly adjusting to maintain equilibrium.
Step-by-step explanation:
The term for keeping a stable internal environment despite changes in the outside environment is homeostasis. This biological process allows an organism to maintain dynamic equilibrium by making constant adjustments. For example, when the hedgehog in the question eats to a certain level of fullness, it is able to maintain its energy level and weight, demonstrating homeostasis.