Conjugation in latin means "to link together", In chemistry, conjugation is a phemomenon where the p-orbitals of an atom in a compound overlaps with the s or d orbitals of the other atoms in the compound, to produce a conjugated system having high stability, especially when there is an alternation of single and multiple bonds in the compound. 

For example, the follwing compound called as 1,3-pentadiene, exhibits conjugation.