In music, the subdominant is the technical name for the fourth degree of the scale. It is called the subdominant because it is the same distance below the tonic that the dominant is above the tonic. In the C major scale (white keys on a piano), the subdominant is the note F; and the subdominant chord uses the notes F, A, and C. In music theory, the subdominant chord is symbolized with the Roman numeral IV if major and iv if minor.
A cadential subdominant chord followed by a tonic chord (the chord of the key of the piece) produces the so-called "plagal"( or "Amen") cadence.
"Subdominant" also refers to a relationship of musical keys. For example, relative to the key of C major, the key of F major is the subdominant. Music which modulates (changes key) often modulates into the subdominant. Modulation into the subdominant key often creates a sense of musical relaxation; as opposed to modulation into dominant (fifth note of the scale), which increases tension.
In sonata form, the subdominant key plays a subordinate though still crucial role: typically, in the recapitulation, there is a section written in the subdominant key, occurring at the point corresponding to the location in the exposition where the music modulated into the dominant key. The use of the subdominant in this location often serves as a way of keeping the rest of recapitulation in the tonic.