Circularity, which most machinists still call roundness, bounds how far a single circular cross section of a feature may depart from a perfect circle. It is a form control, so it takes no datum, and it applies section by section rather than along a length, which is exactly what distinguishes it from cylindricity.
The tolerance zone
In any plane perpendicular to the feature's axis, all points on the surface must lie between two concentric circles whose radial separation is the tolerance value. The circles float, so nothing about position or orientation is being controlled. Each cross section is judged on its own, which is why a tapered or bowed shaft can be perfectly circular everywhere and still fail cylindricity.
When a size tolerance already covers it
A diameter tolerance applied under the envelope principle already limits form to the size tolerance band, so a separate circularity callout only means something if it is tighter than the size tolerance. Adding a circularity value looser than the diameter tolerance is common on inherited drawings and controls nothing at all, while still costing an inspection.
Measuring it
A rotating spindle roundness tester with a probe against the surface is the proper instrument, producing a polar trace. A vee block and an indicator is the workshop approximation and it can be badly misleading, because certain lobing patterns read as zero error in a vee block while being substantially out of round. Say which method applies if the value is tight.
When it matters
Rotating parts where imbalance or runout would be a problem. Seal seats and bearing journals, where the seal or bearing must contact continuously around the circumference. Plug and ring gauges. In most general engineering the size tolerance is sufficient and a circularity callout is unnecessary cost.
Questions people ask about circularity gd&t
Does circularity need a datum?
No. It is a form control and compares the feature only with itself, so a datum reference beside a circularity symbol is a drawing error.
What is the difference between circularity and cylindricity?
Circularity applies to one cross section at a time. Cylindricity applies to the whole surface at once and therefore also limits straightness and taper.
Is circularity the same as roundness?
Yes, in practice. Roundness is the shop term and circularity is the term the drawing standard uses for the same characteristic.
Do I need circularity if I already have a diameter tolerance?
Only if the circularity value is tighter than the size tolerance. Otherwise it controls nothing and still costs an inspection.