Thunder is the standard special move of Robin in the Super Smash Bros. series in Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate. It originates from the Fire Emblem series as a tome that is the weakest form of Thunder magic.
Upon using the move, Robin charges up a bolt of lightning automatically that can be used at any point by pressing the special button again; getting hit at any point cancels the charge, and it can be stored by shielding or rolling.
Thunder can be used by pressing the special button immediately after charging, or it can be charged into the higher-level Thunder spells, Elthunder, Arcthunder, and Thoron. The standard Thunder fires a small bolt of electricity that travels a short distance, Elthunder fires a small ball that travels a moderate distance, Arcthunder fires a large red ball that starts off moving slowly but travels far, and Thoron fires a bolt of electricity that travels across the entire screen. Like all of Robin's moves, Thunder is affected by the durability system; Thunder can be used twenty times before the tome "breaks", Elthunder can be used seven times, Arcthunder four, and Thoron three. The tome reappears after around ten seconds.
On a side note, if Kirby copies Robin, Kirby's voice while calling out the move is different between the English and Japanese versions, similar to Shulk's Monado Arts and Palutena's Autoreticle. The former is closer to the pronunciation of standard English, akin to some of Ness and Captain Falcon's voice clips, while the latter has Japanese pronunciation.
In Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U, the two custom variants for this move are Thunder+ and Speed Thunder. Thunder+ increases the power of each of the Thunder spells, but the tome has less durability (Thunder can be used ten times, Elthunder four, Arcthunder three, and Thoron once), and each move travels more slowly. Speed Thunder decreases the amount of time it takes to charge the move, and each spell travels farther but has less power.