A genetically modified organism (GMO) is any organism whose genetic material has been altered using genetic engineering techniques (i.e., a genetically engineered organism). GMOs are used to produce medications and genetically modified foods, and are widely used in scientific research and the production of other goods. 

A more specifically defined type of GMO is a "transgenic organism." This is an organism whose genetic makeup has been altered by the addition of genetic material from an unrelated organism.

Typically, GMOs' genetic makeup is altered without the addition of genetic material from an other organisms.