The term "genetically modified organism" (GMO) is legally defined by the European Union.
An organism is "genetically modified", if its genetic material has been changed in a way that does not occur under natural conditions through cross-breeding or natural recombination - Article 2 of the EU Directive on the Deliberate Release into the Environment of Genetically Modified Organisms (2001/18/EG).
In individual cases it can be very controversial if an organism has been genetically modified in a way that does not occur “naturally”. The fact that cultivated plants scarcely resemble their wild relatives is an example of dramatic, human-induced genetic modifications that would not be defined as “genetically modified”.
The Release Directive names several techniques that lead to genetically modified organisms:
Mutations normally do not create GMOs as defined by the law - not even when these mutations are induced artificially.
The handling of GMOs - e.g. release, working in closed systems, production facilities, and marketing - is regulated by special laws.