The nominative case is a grammatical case for a noun, which generally marks the subject of a verb, as opposed to its object or other verb arguments. (Basically, it is a noun that is doing something, usually joined (such as in Latin) with the accusative case.)

Explanation
The nominative case is the usual, natural form (more technically, the least marked) of certain parts of speech, such as nouns, adjectives, pronouns and less frequently numerals and participles, and sometimes does not indicate any special relationship with other parts of speech. Therefore, in some languages the nominative case is unmarked, that is, the nominative word is the base form or stem, with no inflection; alternatively, it may said to be marked by a zero morpheme. Moreover, in most languages with a nominative case, the nominative form is the lemma; that is, it is the one used to cite a word, to list it as a dictionary entry, etc.
Nominative cases are found in Latin, Icelandic and Old English, among other languages. English still retains some nominative pronouns, as opposed to the accusative case or oblique case: I (accusative, me), we (accusative, us), he (accusative, him), she (accusative, her) and they (accusative, them). An archaic usage is the singular second-person pronoun thou (accusative thee). A special case is the word you: Originally ye was its nominative form and you the accusative, but over time you has come to be used for the nominative as well.
The term "nominative case" is most properly used in the discussion of nominative-accusative languages, such as Latin, Greek, and most modern Western European languages. Some writers of English employ the term "subjective case" instead of nominative, in order to draw attention to the differences between the "standard" generic nominative and the way it is used in English.
In active-stative languages there is a case sometimes called nominative which is the most marked case, and is used for the subject of a transitive verb or a voluntary subject of an intransitive verb, but not for an involuntary subject of an intransitive verb; since such languages are a relatively new field of study, there is no standard name for this case.