Definitions for "Aufbau principle"
The principle that atomic orbitals are filled one at a time, starting with the orbital that has the lowest energy.
The electronic configuration of a molecule is determined by filling the molecular orbitals two electrons at a time starting from the lowest-energy orbital (or, in other words, the orbitals are filled from the bottom up without leaving any holes!).
a principle stating that as protons are added one by one to the nucleus to build up the elements, electrons are similarly added to hydrogen-like orbitals.