Juxtaposition is an act or instance of placing two things close together or side by side. This is often done in order to contrast the two, to show unlikeness or differences, to note the opposite qualities of the two, etc.
In music it is an abrupt change of elements.
In film the position of shots next to one another is intended to create meaning within the audience's mind.
In literature it occurs when two images that are otherwise not commonly brought together appear side by side or structurally close together - thereby forcing the reader to stop and reconsider the meaning of the text through the contrasting images/ideas/motifs. ex: slouched alertly
Modernist poetry played extensively with juxtaposing images, inserting unrelated fragments together in order to create wonder and interest in readers.
In mathematics, juxtaposition of symbols is commonly used for multiplication: ax denotes the product of a with x.