Definition of Subject

Subject: As a noun:
1. A branch of learning that forms a course of study. In medical school, one of the subjects traditionally studied in the first year is anatomy.
2. A matter that is studied or otherwise considered. Death is a subject often avoided by doctors in talking with patients.
3. The object of observation,, treatment, or research. There are guidelines for the conduct of research involving human subjects at NIH.

The American Psychological Association (APA) has recommended replacing the term "subject" by "participant" to imply that the person consents to the research. And if someone such as a baby cannot consent, the APA recommended the term "individual."