(1) A term used in semantics and grammar as part of an alternative binary characterization of sentence structure to that traditionally found in the subject / predicate distinction; the opposite term is comment. The topic of a sentence is the entity (person, thing, etc.) about which something is said, whereas the further statement made about this entity is the comment. The usefulness of the distinction is that it enables general statements to be made about the relationships between sentences which the subject / predicate distinction (along with other contrasts of this type) obscures. The topic often coincides with the subject of a sentence, but it need not , and, even when it is a subject, it need not come first in a sentence. It is sometimes referred to as the ‘psychological subject’. Topicalization takes place when a constituent is moved to the front of a sentence, so that it functions as topic.
(2) The phrase topic sentence is used in traditional studies of the structure of paragraphs, to refer to the sentence which introduces the paragraph’s theme.