**1. n. [Geology]**

The stratigraphic principle that, in the case of undeformed, flat-lying strata, younger layers are deposited atop older ones, such that the top layer is youngest and underlying layers increase in age with depth. Nicolaus Steno articulated the law of superposition of strata in the 17^{th} century.

**See:**
absolute age,
bed,
geochronology,
relative age,
stratigraphy,
stratum

**2. n. [Well Testing]**

A mathematical technique based on the property that solutions to linear partial equations can be added to provide yet another solution. This permits constructions of mathematical solutions to situations with complex boundary conditions, especially drawdown and buildup tests, and in settings where flow rates change with time.

**See:**
buildup test,
drawdown test