- regular number field
- regular observation
- regular octagon
- regular octahedron
- regular octahedron group
- regular one parameter subgroup
- regular one-parameter subgroup
- regular open set
- regular operation
- regular operator
- regular orbit
- regular ordinal
- regular ordinal number
- regular outer measure
- regular packing
- regular pair
- regular parameterized surface
- regular parametric representation
- regular parametrization
- regular parametrized curve
- regular part
- regular partial differential equation
- regular patch
- regular path expression
- regular pencil of matrices