Synchronize is an ability that a few Pokémon have, and almost all the Pokémon that do have it are Psychic-Type. This ability passes on status conditions inflicted on the Pokémon back to the opponent. This ability only works with the poisoned, paralyzed, and burned conditions. If the user of this ability was badly poisoned, the opponent would have been regularly poisoned; this no longer applies in Generation V, where the opponent is also badly poisoned.
Outside of battle, if the Pokémon with the Synchronize ability is in the lead spot, the chances that the encountered Wild Pokémon will have the same nature as the one in the lead spot will be raised to 50%.
This ability does not work on Pokémon given to you from other characters, such as Castform from the Weather Institute or Eevee from Bebe or resurrected fossil Pokémon. In Generation IV, this ability also does not work on Roaming Pokemon.