This is the third type of
electron orbital. There are five d
orbitals per shell starting with the third, however the 3d is energetically above the 4s orbital (during filling). There are various shapes of p orbital. Three are structured as four lobes in the plane of two axes. The last two are more complex 3 dimensional structures. The d stands for diffuse.
These orbitals are particularly associated with the chemistry of the transition elements.