* Angle: Measured in radians or degrees, but these are not fundamental units, they're ratios of lengths.
* Relative humidity: A percentage, which is a ratio of two quantities with the same units.
* Specific gravity: The ratio of the density of a substance to the density of a reference substance, so units cancel out.
* Refractive index: A ratio of the speed of light in a vacuum to the speed of light in a medium, making it unitless.
* Strain: A measure of deformation, expressed as a ratio of change in length to original length, resulting in no units.
* Poisson's ratio: The ratio of lateral strain to axial strain, making it unitless.
Important Note: Even though dimensionless quantities don't have units in the conventional sense, they can still be expressed with specific numbers or symbols (like radians or percentages).
