In the study of physiological adaptation, an external demand or force that challenges homeostasis is termed stress, while the resulting internal displacement or change within an organism is termed _____.

The provided material distinguishes an external challenge, called **stress**, from the internal displacement that it produces, called **strain**. This distinction helps separate the cause from the animal’s internal response. The terms are related but not interchangeable in the definitions given. **Key takeaway:** Stress is the challenge; strain is the resulting internal disturbance.\n\n**Glossary:**\n- **Stress**: An external force or demand that challenges homeostasis.\n- **Strain**: The internal displacement associated with stress.