(algebra) An integral domain in which every ideal is a principal ideal.

(sometimes proscribed) Alternative spelling of São Tomé and Príncipe

(physics) The quantum number (of an atomic orbital) that specifies the energy of an electron in an atom and, hence, which shell it occupies

(algebra, ring theory) A commutative ring in which every ideal is a principal ideal.

plural of principal ideal domain

plural of principal ideal ring

plural of principal quantum number

(mathematics) The positive square root of a number; the positive number which, when squared, yields another number

plural of principal-agent problem

The problem that a person or entity serving as an agent for another, and able to make decisions on the other's behalf, may be motivated instead to make decisions that benefit themselves.