Research is a "systematic investigation, including research development, testing and evaluation, designed to develop or contribute to generalizable knowledge". 45 CFR 46.102(d)
A human subject is “a living individual about whom an investigator (whether professional or student) conducting research:(i) obtains information or biospecimens through intervention or interaction with the individual, and uses, studies, or analyzes the information or biospecimens; or (ii) obtains, uses, studies, analyzes, or generates identifiable private information or identifiable biospecimens”. 45 CFR 46.102(e)
This means that people are human subjects. Existing data or specimens with identifiable, private information are also human subjects. This includes data that was not collected by the researcher herself or specifically for the study in question, but that can be traced back or identified with the individuals from whom it was collected.
If your activity falls under FDA regulations, note that the FDA definition of human subjects research includes the use of test articles (i.e., drugs or devices) on humans or human specimens, whether identifiable or not CFR Title 21