What is the significance of using the greek letters sigma and pi for naming sigma and pi bonds? ? ?

Is there a reason for this?

What is the significance of using the greek letters sigma and pi for naming sigma and pi bonds? ? ?
Sigma and pi orbitals are the molecular equivalent and named after the atomic s and p orbitals because they have the same symmetry.





The terms s, p, d %26amp; f were named from simple, profound, diffuse and . . . ???





it's been a while since I did this at university! Perhaps someone else knows more?