1. The process of putting one thing on top of something else: Sometimes the superposition of books can result in confusion because it is usually too difficult to find and to pick out the book that is desired.
2. In geology, the principle that in a sequence of sedimentary strata, the oldest layer is located on the bottom and it is followed in turn by successively younger layers, on up to the top of the sequence of layers: During the process of superpositions, sediments are deposited on the sea floor in horizontal layers, parallel to the Earth's surface so that the oldest layer is on the bottom with each of the younger layers resting on top.

Another geological explanation of the principle of superposition states that in a sequence of sedimentary rocks or lavas, each layer is younger than the layer beneath it and older than the one above it.