In biology, homeostasis is the state of steady internal physical and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits. Other variables include the pH of extracellular fluid, the concentrations of sodium, potassium, and calcium ions, as well as the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life.

समस्थापन या होमिओस्तासिस किसी तंत्र का वह गुण है जिसके द्वारा वह अपने आन्तरिक पर्यावरण में आवश्यक परिवर्तन करके ताप, पीएच (pH) आदि को नियत रखता है। तंत्र - दो प्रकार के हो सकते हैं - खुला (ओपेन) या बन्द (क्लोज्ड)।