Dimensionless numbers are mathematical ratios that are used in physics and engineering to describe physical systems or phenomena.  They are typically used to compare physical quantities that have different units, by eliminating the effects of units and focusing on the underlying physical relationships.  These are nondimensional quantity of dimension one and can not be measured on a scale of physical units.  All quantities of dimension one are said to have the SI coherent derived unit "one" with the symbol "1".  It is a pure number, thus always having a dimension of 1.  The number does not change even if the number system you are working in does.  Dimensionless numbers or quantities are used in many disiplines such as chemistry, economics, engineering, mathematics, and physics.

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These dimensionless numbers provide important insights into the behavior of physical systems, and they are widely used in engineering and physics to analyze and optimize the performance of various systems.