In technical use, a standard is a concrete example of an item or a specification against which all others may be measured. For example, there are "primary standards" for length, mass, and other units of measure, kept by laboratories and standards organizations. Officially certified measuring instruments must be checked for accuracy using such standards (or secondary standards made from the primary).
In analytical chemistry a standard is a preparation containing a known concentration of a specified substance. A simple standard may be a dilute solution of the substance; this serves as a reference to calibrate equipment used to measure a sample's composition in terms of elements. For accuracy, the standard concentration may be specially designed to be as close to real samples as possible.
There are also certified reference materials available which contain independently verified concentrations of elements available in different matrices (a matrix is bulk material of the sample, for example blood).