A topic expression is an expression denoting an entity that a proposition is about when it has a topic-comment structure.
topic phrase (CXN) = a referring phrase that refers to the topic, usually applied to a phrase that is distinct in form and role from the subject phrase. Example: in Japanese Nihon wa syuto ga sumiyoi ‘As for Japan, its capital is a good place to live,’ Nihon wa [Japan TOP] is a topic phrase marked by the topic marker wa. ❡ topic (INF ) = in a topic–comment information packaging, the topic is the referent that the comment is predicated about. Example: in Bill is a teacher, being a teacher is predicated about the referent Bill. A topic is a referent; arguably, all referents are topics, although some referents are more topical and others are less topical. (Section 10.1.2)