(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 ring

plural of principal quantum number

plural of principal ideal domain

(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.

plural of editio princeps

(sometimes proscribed) Alternative spelling of São Tomé and Príncipe