A Sample is a single item or specimen selected from a larger population, such as any subset of a population of any medium (air, water, soil, etc.) used to characterize or make inferences regarding that population.

The Sample Size is the number of specimens units to be collected or the relative size of each specimen, volume, weight, etc.

The Sampled Population is the specimens collected or selected from a set location or locations.

The Sampling and Analysis Plan (SAP) is the detailed document created by the EPA. This Sampling and Analysis Plan (SAP) guidance and template is intended to assist

The acronym SEM may refer to any one of the following: 

In Economics and Management:

• Single European Market (or internal market), a single market of the European Union
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A sewage treatment plant is a facility designed to receive the wastewater from domestic sources and to remove materials that damage water quality and threaten public health and safety

-->Statistical control chart

SI is a shortcut for

• -->Site Inspection 
• International System of Units

A solution is a mixture of a solvent and a solute. In some solutions, such as sugar water, the substances mix so thoroughly that the solute cannot be seen.

A specific conductance is a measure of the ability of water to conduct an electrical current as measured using a 1-cm cell and expressed in units of electrical conductance, i.e., Siemens per centimeter at 25 degrees Celsius.