A seventh chord is a type of chord that consists of four notes: the root, third, fifth, and seventh. It adds an extra note to the basic triad, which includes only three notes. This additional note creates a richer and more complex sound, often used in various music genres to enhance harmony. Seventh chords can be categorized into different types, such as major seventh, minor seventh, and dominant seventh. Each type has a unique sound and function within a musical context, making them essential for composers and musicians when creating progressions and adding emotional depth to their music.