Illustrated Glossary of Organic Chemistry

Partial pi bond: A pi bond interaction between two adjacent p orbitals that is absent in some resonance contributors and present in others. The net effect is bond with some pi bond characteristics, but not the strength or structure of a full and complete pi bond. A partial pi bond arises from pi electron delocalization over three or more atoms. A partial double bond is indicated with two parallel lines (like a normal double bond); one solid line and one dashed line. A partial pi bond can also cause a partial triple bond. A partial double bond has a bond order between one and 2; a partial triple bond has a bond order between 2 and 3.

Acetate ion resonance contributors
Acetate ion resonance hybrid

Kekule structures for benzene
Benzene resonance hybrid

Acylium ion resonance contributors   
Acylium ion resonance hybrid