Illustrated Glossary of Organic Chemistry

Pi* orbital (π* orbital): The antibonding molecular orbital component of a pi bond.

The π* orbital of ethylene's carbon-carbon pi bond has four orbital lobes (two orbital lobes on each sp² carbon atom). It is an antibonding molecular orbital. This π* orbital has two nodes: one node is the plane which contains the atoms, and the other node is a plane perpendicular to this, between the two carbon atoms.

The π orbital of ethylene's carbon-carbon pi bond has two orbital lobes, one above the plane of the atoms, and another below the plane. This is a bonding molecular orbital. This same plane is the pi orbital's one node.