A tableau is a large picture on stage that is formed from an artistic grouping or formation. Very typically in full length ballets, a tableau can be the start of an Act or the finish of one. It is usually an important moment frozen in time in which the audience can fully registered what they just saw.
In both classical and contemporary ballets, a tableau can be used. There are many iconic tableaux that are very recognizable and that have become famous in the ballet world. One example is the opening of Balanchine’s Serenade in which the 17 women dancers are on stage with their right hands flexed upward and are awash in blue lighting that makes it seem like they are standing in the moonlight. They are in this pose when the curtain comes up and immediately the audience knows that this picturesque scene they’re viewing is of importance and should be remembered. There are many examples of tableaux is just Serenade alone, like in the Elegy movement when the Waltz Girl is on the floor with her hand up to the Elegy man who is behind her and the Dark Angel is standing directly behind the man with her arms out like she is about to take flight.
An example of a tableau in classical ballet is the end of the first of Giselle. After the Mad Scene, Giselle dies, the remaining dancers on stage create a picture surrounding Giselle being held by her mother. This tableau is one of extreme sadness and mourning because the young Giselle has died of a broken heart.