The word subject has several meanings:

1. The topic or main idea of something. When your teacher asks what subject you're studying, she wants to know whether it's math, science, history, or another topic. A book's subject might be dinosaurs, adventure, or friendship. If someone keeps talking about the same thing over and over, you might say they won't change the subject.
2. A person or thing being discussed, studied, or experimented on. Scientists might study their subjects to learn how people respond to different situations. An artist's subject is whatever they're painting or drawing, whether it's a bowl of fruit, a landscape, or a person posing for a portrait.
3. In grammar, the part of a sentence that performs the action. In “The dog chased the ball,” the dog is the subject because it's doing the chasing. Every complete sentence needs a subject and a verb.
4. A person under the authority of a king or queen. The subjects of Queen Elizabeth I lived under her rule. Citizens of a democracy aren't subjects because they have a voice in their government rather than simply obeying a monarch.

The word can also be a verb meaning to cause someone to experience something, usually something unpleasant: “The teacher subjected us to a pop quiz.”