出血是指血液从血管或心脏外出至组织间隙、体腔或身体表面。

根据出血原因可分为两种：心脏或血管壁破裂的出血称为破裂性出血；毛细血管和细静脉壁通透性增高的出血称为漏出性出血。

根据出血部位又可分为两种：内出血是血液从血管流到血管外的体内；外出血是血液从血管通过身体表面的伤口流到体外，亦包括通过身体本来已有的开口诸如口、鼻、耳、尿道、阴道、或肛门流出血液。

Bleeding, technically known as hemorrhaging or haemorrhaging (see American and British spelling differences), is blood escaping from the circulatory system. Bleeding can occur internally, where blood leaks from blood vessels inside the body, or externally, either through a natural opening such as the mouth, nose, ear, urethra, vagina or anus, or through a break in the skin. Hypovolemia is a massive decrease in blood volume, and death by excessive loss of blood is referred to as exsanguination. Typically, a healthy person can endure a loss of 10–15% of the total blood volume without serious medical difficulties (by comparison, blood donation typically takes 8–10% of the donor's blood volume). The stopping or controlling of bleeding is called hemostasis and is an important part of both first aid and surgery.