noun

• A value kept constant during an experiment, equation, calculation, or similar, but varied over other versions of the experiment, equation, calculation, etc.
• (sciences) A variable that describes some system (material, object, event, etc.) or some aspect thereof.
• (statistics) Any measured quantity of a statistical population that summarises or describes an aspect of the population.
• (machine learning) A variable of a model that is trained by a machine learning algorithm.
• (programming) An input variable of a function definition, that gets an actual value (argument) at execution time.
• (programming, loosely) An actual value given to such a formal parameter.
• A characteristic or feature that distinguishes something from others.
• (geometry) In the ellipse and hyperbola, a third proportional to any diameter and its conjugate, or in the parabola, to any abscissa and the corresponding ordinate.
• (crystallography) The ratio of the three crystallographic axes which determines the position of any plane.
• (crystallography) The fundamental axial ratio for a given species.