variable, x

The quantity or characteristic measured or computed. The corresponding numerical value may be taken for statistical handling; it may, for example, be a measured value or result.
Note that statistical usage employs capital letters for random variables, and lower case for particular or observed values. In circumstances where the choice of 'x' as the symbol may cause confusion, another symbol may be used.
See also: dependent and independent variables
PAC, 1994, 66, 595 (Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations 1994)) on page 597