A geometric tolerance that specifies how closely a physical feature approaches a mathematically perfect circle. It is measured by rotating a part against a highly sensitive stylus or using optical comparators to map the profile. Errors in roundness, such as lobing, directly increase the measurement uncertainty when determining the diameter of a cylinder.
Heard on the job
The micrometer says the diameter is in spec, but the roundness is out because of a three-lobe shape from the chuck.