A medical device is defined as any “instrument, apparatus, implement, machine, appliance, implant, reagent for in vitro use, software, material or other similar or related article” that is intended for medical purposes (but not by pharmacological, immunological or metabolic means).
According to the International Organization for Standardization (ISO), these medical purposes include:
diagnosis, prevention, monitoring, treatment or alleviation of disease
diagnosis, monitoring, treatment, alleviation of or compensation for an injury
investigation, replacement, modification, or support of the anatomy or a physiological process
supporting or sustaining life
control of conception
disinfection of other medical devices
providing information by means of in vitro examination
Additionally, in some regions, the following may also be considered medical devices:
aids for persons with disabilities
devices incorporating animal and/or human tissues
devices for in vitro fertilization or assisted reproduction technologies.
I constantly run into people who believe that an app or piece of software can’t be a medical device. This is a wrong and dangerous assumption (see glossary entry on “Software as a Medical Device”).
For the source and more information refer to ISO 13485: 2016.