Vitamin D is a fat-soluble vitamin that involved mainly for intestinal absorption of calcium with the regulation of calcium/phosphorus hemostasis, bone formation and remodeling, and anti-inflammatory with immune-modulating advantages. Insufficient solar UVB irradiation and/or lack of vitamin D in diet could induce a deficiency of vitamin D in animals and humans that manifested by poor appetite and growth and consequently leads to osteomalacia and osteodystrophy. A wide range of non-skeletal disorders, neoplastic, cardiovascular, and auto-immune diseases are attributed to vitamin D deficiency. This current review is presenting an overview of the origin/source, metabolism and activation, biology, physiology and functions, and the pathology of vitamin D in human and domestic animals. Special topics of vitamin D deficiency, toxicity, and rickets are also detailed in this review. In conclusion, maintenance of a normal serum level of active form of vitamin D prevents the occurrence of broad ranges of disorders and diseases. Beside enhance the weight gain and the growth of the growing animals, normal levels of vitamin D may also play a crucial role as immune-modulating factors for making animals and people resistant against contagious diseases such as hemorrhagic septicemia and coronavirus (COVID-19).