1, 4. Subject, theme, topic are often interchangeable to express the material being considered in a speech or written composition. Subject is a broad word for whatever is treated in writing, speech, art, etc.: the subject for discussion. Theme and topic are usually narrower and apply to some limited or specific part of a general subject. A theme is often the underlying conception of a discourse or composition, perhaps not put into words but easily recognizable: The theme of a need for reform runs throughout her work. A topic is the statement of what is to be treated in a section of a composition: The topic is treated fully in this section.

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