The word "conjugation" is derived from a Latin word that means "to link together".  In organic chemistry, it is used to describe the situation that occurs when p systems are "linked together".

• An "isolated" p system exists only between a single pair of adjacent atoms (e.g. C=C)
• An "extended" p system exists over a longer series of atoms (e.g. C=C-C=C or C=C-C=O etc.).
• An extended p system results in a extension of the chemical reactitvity.

• A conjugated system requires that there is a continuous array of "p" orbitals that can align to produce a bonding overlap along the whole system.
•  If a position in the chain does not provide a "p" orbital or if geometry prevents the correct alignment, then the conjugation is broken at that point.

    You can investigate these differences by studying the following examples, pay particular attention to the "p" orbitals:
 

System	CHIME	Line diagram	p system	Type
ethene				isolated
propene				isolated
1,2-propadiene (allene)				cumulated
1,3-butadiene				conjugated
1,3-pentadiene				conjugated
1,4-pentadiene				isolated
1,3-cyclopentadiene				conjugated
1,3-cyclohexadiene				conjugated
1,4-cyclohexadiene				isolated
benzene				conjugated

    The result of conjugation is that there are extra p bonding interactions between the adjacent p systems that results in an overall stabilisation of the system.