Is it accurate to define homeostasis as a constant internal environment

No quite ... Homeostasis is the maintenance of constant internal environment

For example the constant internal temperature of our body is 37 degrees Celsius or 98.6 degrees Fahrenheit. If our body increases or decreases some changes happens to your body in order to return the normal body temperature. If your freezing your blood vessels near the skin constricts so that the heat running through your blood doesnt leave your body because you are in need of heat.