A diminished seventh chord is a type of chord made up of four notes, each a minor third apart. It consists of a root note, a minor third, a diminished fifth, and a diminished seventh. This chord has a tense and unstable sound, often used to create tension in music. Diminished seventh chords can be found in various musical genres, including classical, jazz, and popular music. They are commonly used to transition between chords or to lead into a resolution, making them an essential tool for composers and musicians seeking to enhance emotional expression in their work.