Mediant: The mediant is the third scale degree of a major or minor key. It often serves as an intermediary between tonic and dominant, adding emotional depth.
Subdominant: The subdominant is the fourth scale degree of a major or minor key. It provides a sense of stability and prepares for the return to the tonic.
Subtonic: The subtonic is another term for the seventh scale degree in a major key. It differs from the leading tone because it doesn't have strong tension towards resolving to the tonic.