Synergy! was a secret achievement in Plants vs. Zombies Adventures. It was earned by using five different types of plants in a level. Synergy! could be easily earned once the fifth seed slot or a VIP plant was unlocked. It was impossible if the player has not unlocked more than four seed slots, excluding the VIP slot.
The word "synergy" means the interaction or cooperation of two or more organizations, substances or other agents to produce a combined effect greater that the sum of their separate effects.