1. The fourth scale degree in the Western major or minor scales. 2. The major or minor chord built on the fourth scale degree ("subdominant chord"). 3. The harmonic function associated with the subdominant chord.
In music, the subdominant is the technical name for the fourth tonal degree of the diatonic scale. It is so called because it is the same distance "below" the tonic as the dominant is above the tonic - in other words, the tonic is the dominant of the subdominant. It is also the note immediately "below" the dominant http://dictionary.reference.com/search?q=subdominant.