生物系统层级关系：生物圈 > 生态系统 > 群落 > 种群 > 个体

生物（英语：Organism，又称有机体）是指称类生命的个体。在生物学和生态学中， 地球上约有870万个物种（±130万），其中650万种物种在陆地上，220万种生活在水中。生物最重要和基本的特征在于生物进行新陈代谢及遗传，所有生物一定会具备合成代谢以及分解代谢，这是相反的两个过程，并且可以繁殖下去，这是类生命现象的基础。

生命的起源和生命各个分支之间的关系一直存在争议。一般而言，生物分为两大类：原核生物和真核生物。原核生物分为两个域：细菌（Bacteria）和古菌（Archaea），这两个域相互之间的关系并不比他们和真核生物的关系更为接近。在演化史的研究上，原核生物和真核生物之间一直缺乏联系。

In biology, an organism is any contiguous living system, such as an animal, plant, fungus, or bacterium. All known types of organisms are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. An organism consists of one or more cells; when it has one cell it is known as a unicellular organism; and when it has more than one it is known as a multicellular organism. Most unicellular organisms are of microscopic size and are thus classified as microorganisms. Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs.