What term describes the relatively constant internal physical conditions of an organism?

The answer is homeostasis.

Homeostasis is a mechanism which controls internal conditions of an organism so that internal conditions remain stable without regard to the external conditions. So, it regulates body temperature, water level, sugar level, etc. It works through negative feedback mechanism. For example, if body temperature is increased, the body detects the change and activate mechanisms that decrease body temperature. Without homeostasis, the organisms could not survive.