1. (literary theory) The central, dominant, or unifying idea in a text (or several texts), or a standpoint on the subject, implicit or explicit, which is reflected in recurrent motifs, imagery, and/or symbols.
2. More loosely, the general topic of a text or group of texts.
3. In discourse analysis and systemic-functional linguistics, the part of a sentence that indicates the topic: what the utterance is about. This is often the initial element. The rheme is the semantically important part of an utterance communicating information relating to the theme. Some theorists also refer to an intervening part (e.g. a verb) as a transition or a transitional element. For example, in ‘The medium is the message’, ‘The medium’ is the theme, ‘is the message' is the rheme, though some would list ‘is’ as the transition. The distinction is sometimes related to the concepts of given and new.