What is the 2 or 2nd Scale Degree?

The 2nd scale degree is the second note of a key above the root. In the key of C, the 2nd scale degree would be D. In the key of G, the second scale degree would be A.

When used in a chord, the 2 can act as a suspension by replacing the 3. Thus, a "C2" chord would be contain a C, D, and G. Often the D resolves up to an E or an E flat creating either a major or minor chord respectively. "2 chords" have a beautiful and rich dissonance. 

When this note is used an octave higher it is then referred to as a "9" or a "9th." The interval of a ninth is dissonant but has a smoother sound than when the note appears directly above the root.