Dictionary.net

(geometry) the placement of one object ideally in the position of another one in order to show that the two coincide

The act of superposing, or the state of being superposed; as, the superposition of rocks; the superposition of one plane figure on another, in geometry.

Act of superposing: state of being superposed: that which is above anything.

n. A being placed or situated above or upon something :-that which is situated above or upon something else; -in geology, the order in which mineral strata are placed upon or above other strata.