What the law says:
"data subject" means an individual who is the subject of the personal data.
What it means:
The data subject is the living natural person the information is about or relates to.
A data subject does not necessarily have a direct relationship with the data controller. For example, CCTV is often installed as a crime prevention measure. If images of an individual walking past premises are captured on CCTV these images are the personal data of that individual.