Non-dimensional is a term used in mathematics and physics to describe a quantity that has no physical dimensions or units. It is often used in calculations and equations to simplify the problem or make it easier to understand. Some synonyms for non-dimensional include dimensionless, unitless, and scale-invariant. These terms all refer to the same concept of a quantity that has no specific unit of measurement. Other related terms include dimensionality reduction, which is the process of simplifying a problem by reducing the number of dimensions involved, and normalization, which is the process of scaling variables to have similar ranges.