Illustrated Glossary of Organic Chemistry

Orbital: A mathematical equation which defines a volume of space in which there is a specified probability of finding an electron of a specified energy. Shown below are plots of orbital equations.
An s atomic orbital is spherical.
It has no nodes.		   
A p atomic orbital has two orbital lobes
and a node at the nucleus. These px, py,
and pz orbitals are orthogonal.		   
The π1 molecular orbital of
1,3-butadiene has one node.