Plus

 and Minus are Pokémon abilities introduced in the third generation. It boosts Special Attack when an ally out has one of these abilities.

Description
Originally, Plus and Minus only powered up when in a Double Battle with an ally with opposite ability. It raises the Special Attack of both Pokémon by 50%

In the Fifth Generation, the special attack boost works with allies with either ability regardless of what ability the Pokémon has. The move Magnetic Flux increases the Defense and Special Defense of a Pokémon with Plus or Minus by one stage and the move Gear Up increases the Attack and Special Attack of a Pokémon with Plus or Minus by one stage.

Pokémon with Plus

 * Plusle
 * Klink
 * Klang
 * Klinklang

As a Hidden Ability

 * Mareep
 * Flaaffy
 * Ampharos
 * Dedenne

Pokémon with Minus

 * Minun
 * Klink
 * Klang
 * Klinklang

As a Hidden Ability

 * Electrike
 * Manectric