干细胞(Stem Cell)是指原始的且未特化的细胞，可以通过有丝分裂不断增殖，在特定条件下可以分化为特定的组织和器官，理论上可以再生新的机体的潜能。干细胞也称多能细胞，存在于各类多细胞组织里，具有经由有丝分裂与分化来分裂成多种的特化细胞的能力，而且可以利用自我更新来提供更多干细胞。对哺乳动物来说，干细胞分为三大类：胚胎干细胞 ，诱导多能干细胞和 成体干细胞。胚胎干细胞取自囊胚里的内细胞团，具有分化为新的机体的可能;诱导多能干细胞来源于成体细胞重编程而获得。而成体干细胞则来自各式各样的组织。成体干细胞在机体损伤修复中发挥这至关重要的作用。
Stem cells are undifferentiated biological cells that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells. They are found in multicellular organisms. In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm (see induced pluripotent stem cells)—but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.