Juxtaposition
Placing two contrasting or unexpectedly related elements together in a frame so each comments on the other.
In depth
Juxtaposition creates meaning that neither element carries alone, which makes it the central device of street and documentary photography. It depends entirely on viewpoint and timing, since the relationship exists only from one position at one moment. Its risk is being too neat — a joke that closes the picture rather than opening it.
Examples in practice
A billboard model's serene face above a weary commuter creates a comment neither element makes by itself.