superposition principle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A principle holding that two or more solutions to a linear equation or set of linear equations can be added together so that their sum is also a solution.
• noun A principle holding that two or more states of a physical system can be added together to create an additional state.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics, physics The principle that a linear combination of two or more solutions of an equation is itself a solution; it is a feature of many physical laws

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (geology) the principle that in a series of stratified sedimentary rocks the lowest stratum is the oldest