A possessive adjective is a word that shows ownership or relationship. It modifies a noun by indicating to whom it belongs. Common possessive adjectives include my, your, his, her, its, our, and their. For example, in the phrase "my book," the word "my" tells us that the book belongs to the speaker. Possessive adjectives are used before nouns and help clarify who possesses something. They are different from possessive pronouns, which replace nouns instead of modifying them. For instance, in "That book is mine," "mine" is a possessive pronoun, while "my" in "my book" is a possessive adjective.