PlusPower attaches to a Pokémon for a single turn, boosting the damage of its next attack by 10 before the card discards itself. The original Base Set wording applied this bonus after Weakness and Resistance were calculated, while the Diamond and Pearl wording changed the timing to apply the bonus before Weakness and Resistance, which changes the total against a Pokémon with Weakness or Resistance. The Black and White wording dropped the attachment, instead giving your Pokémon's attacks 10 more damage for the turn.
PlusPower first appeared in Base Set in 1999 with art by Keiji Kinebuchi. Ryo Ueda illustrated the Diamond and Pearl era reprints and 5ban Graphics the Black and White printing and its trainer kit reprints. The Unleashed printing briefly changed 'Active Pokémon' to 'Defending Pokémon' before an errata reverted it. It has also reprinted in BW trainer Kit products for Excadrill and Zoroark during the Black and White era, following its long run through Base Set 2, Diamond and Pearl, Secret Wonders, Platinum, and Unleashed.
PlusPower is the Japanese name of X Attack, an item in the games; the English card may have used the Japanese name by mistake, or because 'PlusPower' is inscribed on the object in the card's artwork. PlusPower's Diamond and Pearl printing originated from the Japanese Space-Time Creation expansion and was reprinted across Secret Wonders and Platinum.
Base Set printing is PlusPower's first appearance, illustrated by Keiji Kinebuchi
Diamond and Pearl printing changes the damage bonus to apply before Weakness and Resistance
Unleashed printing briefly used 'Defending Pokémon' wording before an errata reverted it
Diamond and Pearl's wording applied the extra 10 damage before Weakness and Resistance, instead of after as in the original Base Set wording.
It is the Japanese name of the video game item X Attack, possibly used on the English card by mistake or because it appears inscribed in the artwork.
No, the switch to 'Defending Pokémon' was reverted by errata back to the Diamond and Pearl wording.
From Base Set (1999) · 2 printings
Attach PlusPower to your Active Pokémon. At the end of your turn, discard PlusPower. If this Pokémon's attack does damage to the Defending Pokémon (after applying Weakness and Resistance), the attack does 10 more damage to the Defending Pokémon.
From Diamond & Pearl (2007) · 3 printings
Attach PlusPower to 1 of your Pokémon. Discard this card at the end of your turn. If the Pokémon PlusPower is attached to attacks, the attack does 10 more damage to the Active Pokémon (before applying Weakness and Resistance).
From Unleashed (2010) · 1 printing
Attach PlusPower to 1 of your Pokémon. Discard this card at the end of your turn. If the Pokémon PlusPower is attached to attacks, the attack does 10 more damage to the Defending Pokémon (before applying Weakness and Resistance).
From Black & White (2011) · 3 printings
During this turn, your Pokémon’s attacks do 10 more damage to the Active Pokémon (before applying Weakness and Resistance).

PlusPower
Item
First printed in Base Set (1999)
During this turn, your Pokémon’s attacks do 10 more damage to the Active Pokémon (before applying Weakness and Resistance).