In ballet, a pas de deux (French, step/dance for two) is a duet in which ballet steps are performed together. It usually consists of an entrée, adagio, two variations (one for each dancer), and a coda.
Well, doesn't that definition make the PDD seem so stale? A truly great PDD is full of emotion, strength, precision, and the incredible connection shared between the two dancers on stage.
This picture explains it better than I ever could.