I�m only an amateur linguist, but from a linguistic point of view, there are several related terms in this space.
The term �subject� has many meanings in English, including �topic�. But from a grammatical and linguistic standpoint, there are only two meanings of �subject�, and �topic� is a distinct term. One meaning of �subject� is specific to the grammar of a given language, such as English, in which we have �subjects� and �objects�. The more general linguistic term is usually capitalized (�Subject�) and even more usually abbreviated as �S�; it refers only to what we call the subject of an intransitive verb. What we call the subject of a transitive verb is more generally called the �Agent�, or �A�, and what we call the direct object of a transitive verb is more generally called a �Patient�, or �P�. Different languages group S, A, and P differently in how they are treated by the grammar; English, like most European languages, lumps S and A together and separates P; such languages are called �nominative-accusative� (or just �accusative�) languages. Then you have �topic� and �focus�, which are closely related. Basically, the focus is the main thing that you�re talking about in a given sentence, and is usually introduced by that sentence. The topic is a known quantity under discussion. So you frequently have the focus of one sentence � which may or may not be its subject - being the topic for subsequent discussion. Example: A) I went to the game yesterday. B) While there, I saw Smoltz strike out 15 batters. The grammatical subject of sentence A is �I�, and it is actually a Subject in SAP terms. But the focus is the game. The definite article implies that the listener knows which game is being discussed, but it�s still being introduced as a new element here. If there is a topic, we don�t know what it is. The grammatical subject of B is still �I�, but this time it�s an Agent; the Patient is an action (Smoltz striking out 15 batters), and that action is also the focus. But the topic is the game.
