The genitive is a grammatical case used to indicate possession or a relationship between nouns. In English, it is often formed by adding an apostrophe and "s" to a noun, such as in the phrase "John's book," which shows that the book belongs to John. In many languages, the genitive case can also express other relationships, such as origin or description. For example, in Latin, the genitive form of a noun can indicate that something is part of or related to another noun, like "the city's history," where the history is associated with the city.