What is dimensionless?

Dimensionless refers to quantities that do not have any associated physical units, meaning they are pure numbers without dimensions such as length, mass, or time. Examples include ratiOS like the coefficient of friction, the Reynolds number in fluid dynamics, or percentages. These quantities often arise in mathematical equations and models, allowing for comparisons and scaling without the influence of specific units. Dimensionless numbers are particularly useful in physics and engineering for simplifying complex relationships.