1. generally, the individual human or nonhuman animal that takes part in an experiment or research study and whose responses or performance are reported or evaluated; less frequently, the “subject” may also be an institution, group, or other entity. Many now recommend that the term participant be used for humans, considering the word subject to be depersonalizing and to imply passivity and submissiveness on the part of the experimentee.

2. in linguistics, the principal noun phrase in a clause or sentence about which something is stated (compare predicate). The subject is usually (but not always) the agent of the main action and in English can usually be identified as the noun phrase whose number and person govern the form of the main verb.

3. an area or branch of knowledge or a course of study.