Key Difference:  A topic defines a particular aspect that will be covered in conversation or in any written form, whereas subject is used in context to a wider aspect of knowledge. A subject cannot be a part of a topic whereas a subject may have many topics covered in it.  However, topic and subject may be used interchangeably if they are used to denote the important aspect of a document, lecture, discussion, presentation or a conversation.

A topic and subject seems to be similar, and many of us might be using them interchangeably. However, there is a slight difference between a topic and subject. Let us define some examples and then relate them to their definitions.

Suppose that a teacher asked the students to write articles on various threats to environment. The student’s got excited with the article writing and next day submitted the articles. Mary had written an article on the air pollution, whereas Sally had written about the water pollution. Similarly, other students had also written the articles.

Now, the teacher is impressed with Marry’s and Sally’s article, thus she narrates them to the children. Now, can you see the relation between the environment threat and air pollution or between the environment threats and water pollution. The relation may solve your doubts about the difference between a topic and subject. A subject is often related to a lager scale than a topic. A subject refers to a particular field of knowledge and can be composed of various topics. In this example, the subject of writing articles was the environmental issues, and various students came up with different topics on this particular subject matter.

Again Marry had written about various types of air pollutants. Each of them had been elaborated nicely by her. Now, in this context the subject matter is the air pollution and the topics are various elaborated types of air pollutants.

 This is the reason that many times they are used interchangeably. They only differ when the subject within the subject needs to be defined and then the inner subject becomes the topic. Subject deals with a broader spectrum, whereas topic is more about narrowing the spectrum and concentrating on a particular aspect.

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