जीव (Organism) शब्द जीवविज्ञान में सभी जीवन-सन्निहित प्राणियों के लिए प्रयुक्त होता है, जैसे: कशेरुकी जन्तु, कीट, पादप अथवा जीवाणु। एक जीव में एक या एक से अधिक कोशिकाएँ होती हैं। जिनमें एक कोशिका पाया जाता है उसे एक कोशिकीय जीव कहते हैं; एक से अधिक कोशिका होने पर उस जीव को बहुकोशिकीय जीव कहा जाता है।

An organism is defined in a medical dictionary as any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what is an organism. Among the commonest is that an organism has autonomous reproduction, growth, and metabolism. This would exclude viruses, despite that fact that they evolve like organisms. Other problematic cases include colonial organisms; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation, with some insects reproducing, others not, like cells in an animal's body. The body of a siphonophore, a jelly-like marine animal, is composed of organism-like zooids, but the whole structure looks and functions much like an animal such as a jellyfish, the parts collaborating to provide the functions of the colonial organism.