I¹m only an amateur linguist, but from a linguistic point of view, there are several related terms in this space.
The term ³subject² has many meanings in English, including ³topic². But from a grammatical and linguistic standpoint, there are only two meanings of ³subject², and ³topic² is a distinct term. One meaning of ³subject² is specific to the grammar of a given language, such as English, in which we have ³subjects² and ³objects². The more general linguistic term is usually capitalized (³Subject²) and even more usually abbreviated as ³S²; 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 ³Agent², or ³A², and what we call the direct object of a transitive verb is more generally called a ³Patient², or ³P². 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 ³nominative-accusative² (or just ³accusative²) languages. Then you have ³topic² and ³focus², which are closely related. Basically, the focus is the main thing that you¹re 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 ³I², 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 it¹s still being introduced as a new element here. If there is a topic, we don¹t know what it is. The grammatical subject of B is still ³I², but this time it¹s 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.
