The subject of a verb is the argument that generally refers to the origin of the action or the undergoer of the state shown by the verb. However, this definition depends on the particular language under consideration. In languages where a passive voice exists, the subject of a passive verb may be the target or result of the action. This is a semantic definition.

An alternative definition would be: the subject of an intransitive verb is its only argument; the subject of a transitive verb is its main argument. Transitive verbs usually agree with the subject, if such agreement exists at all. This is a morphosyntactic definition.

The subject most often carries the least-marked case; that is, in a language that marks morphological case on the arguments of a proposition, the subject tends to be marked with the least salient morphology, or is left unmarked.

In addition, the subject tends to come first of the two core arguments of a transitive verb; only a minority of languages place the subject after the object.

Finally, the subject tends to be the topic of the proposition. In languages with no other means to mark a topic, making an object into a subject by using passivization (I did it → it was done) is a way to topicalize said object. (See also topic-prominent languages.)

There are languages where a verb is allowed to have no arguments, and therefore no subject (null subject language) These cases must be distinguished from those where the subject can be dropped and left implicit, referenced by agreement in the verb, or simply to be guessed from context (pro-drop language).

In English, verbs actually have two subjects: the semantic subject, which is the doer of the verb according to meaning, and a syntactical subject, which is what the verb agrees with, and it determines which case a pronoun gets. In most cases, they are identical. For example, in the sentence: I edit articles, "I" is the semantic subject because I actually am editing letters, and I is the syntactical subject because the verb agrees with it. However, in the sentence: Articles are edited by me, "articles" is the syntactical subject because the verb agrees with it. "Articles" is still the semantic object. "Me" is the syntactical object because it is in the accusative case, but it is still the semantic subject because I, not articles, am doing the editing.

In some languages, like English or French, verbs must always have a syntactical subject, either a noun or noun phrase, or a pronoun, even if there is no semantic subject. This is why verbs like rain must carry a "subject" such as it, even if nothing is actually "doing" the raining. It is in this case an expletive and a dummy pronoun.