A possessive or ktetic form (abbreviated POS or POSS; from Latin: possessivus; Ancient Greek: κτητικός, romanized: ktētikós) is a word or grammatical construction used to indicate a relationship of possession in a broad sense. This can include strict ownership, or a number of other types of relation to a greater or lesser degree analogous to it.

Most European languages feature possessive forms associated with personal pronouns, like the English my, mine, your, yours, his and so on. There are two main ways in which these can be used (and a variety of terminologies for each):

• Together with a noun, as in my car, your sisters, his boss. Here the possessive form serves as a possessive determiner.
• Without an accompanying noun, as in mine is red, I prefer yours, this book is his. A possessive used in this way is called a substantive possessive pronoun, a possessive pronoun or an absolute pronoun.

Some languages, including English, also have possessive forms derived from nouns or noun phrases, such as Jane's, cows' and nobody else's. These can be used in the same two ways as the pronoun-derived forms: Jane's office or that one is Jane's.

Possessives are sometimes regarded as a grammatical case (the possessive case), although they are also sometimes considered to represent the genitive case, or are not assigned to any case, depending on which language is being considered. On the other hand, some languages, such as the Cariban languages, can be said to have a possessed case, used to indicate the other party (the thing possessed) in a possession relationship. A similar feature found in some languages is the possessive affix, usually a suffix, added to the (possessed) noun to indicate the possessor, as in the Finnish taloni ("my house"), where talo means "house" and the suffix -ni means "my".

The concepts of possessive forms and genitive forms are sometimes conflated, although they are not exactly the same. The genitive form, which does not exist in modern English as a productive inflection outside of pronouns (see below), represents an of relationship, which may or may not be possessive; in other words, the possessive is a subset of genitive. For example, the genitive form "speed of the car" is equivalent to the possessive form "the car's speed". However, the genitive form "pack of dogs" is not the same as the possessive form "dogs' pack" (though it is the same as "dog pack", which is not possessive).