- principal meridian line
- principal minors of a matrix
- principal moment of force system
- principal normal vector
- principal of polarity in plane
- principal operators
- principal osculating quadric
- principal parameter
- principal part of an infinitesimal
- principal plane of a cube
- principal point
- principal polarization
- principal polar subgroup of a lattice-ordered group
- principal realization of basic representation
- principal series
- principal strain
- principal stress
- principal symbol of partial differential operators
- principal symmetric plane of a cube
- principal tangent curve
- principal ultrafilter
- principal value interval of inverse trigonometric function
- principal value of arguments of complex numbers
- principal value of inverse trigonometric function
- principal value of the logarithmic function