suhb jekt

noun
definition:  the topic of what is said, written, studied, or the like.
example:  Her new novel is about an interesting subject.
example:  A great deal of research has been done on that subject.

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Collocations:  Words often used in combination with the noun “subject”

PREPOSITION + subject:    about~  (e.g., a film about a serious subject),  on~  (e.g.,  A great deal has been written on that subject)

Difference note

As you can see from the collocations above, both the prepositions “on” and “about” are used with the word “subject.”  The meaning of these prepositions is the same or nearly the same, but they often introduce different ideas and are usually used in different contexts.

We tend to use “about” when the “subject” is a topic of ordinary conversation or an informal focus of a typical movie or book, as opposed to a documentary or formal reporting of research (e.g., Let’s talk about another subject besides your ex-wife; It’s a great book about the subject of love within a family).  We tend to use “on” when the “subject” is a field of study or wide, general topic that has been investigated (e.g., She lectured on the subject of kinship terms;  It’s an exposé on the subject of violent crime.)  “On” tends to be used when there has been some research done or study made in connection with a subject, or when there has been or will be in-depth discussion of the subject.