The term "subject" refers to the main focus or topic of discussion in a sentence or a piece of writing. In grammar, the subject is typically the noun or pronoun that performs the action of the verb. For example, in the sentence "The dog barks," the dog is the subject because it is the one doing the barking. In a broader context, "subject" can also refer to an area of study or a theme in various fields, such as science, literature, or art. For instance, in a classroom setting, students might study different subjects like mathematics or history, each focusing on specific knowledge and skills.