Pas de deux is a dance of two (usually main) characters in classic ballet. Derived from French language, the term literally means “the step of two”.
A classic version of pas de deux includes 5 parts: entrée, adagio, ballerina’s and danseur’s variations, coda. Entrée is the beginning, during which the dancers enter the stage, apprising audience of the forthcoming dance. Coming next is adagio, the actual dance of the two characters fluidly demonstrating their matched technique along with the relationship between them. Adagio is followed by successive solo variations of the ballerina and the danseur before they once again step on the stage together to perform the concluding part of the piece, dynamic and varied, coda. It often marks not only the end of the composition, but also the end of the play. However, if the two dancers on the stage only dance together or for each other, not following the order above, you are probably looking at a duet, not pas de deux.
There exist other similar to pas de deux forms of ballet compositions like pas de trois, pas de quatre and even pas classique, which features corps de ballet performing along with the soloists.