Lightning Kick is the name for Zelda's forward aerial and back aerial attacks in Super Smash Bros. Melee and Super Smash Bros. Brawl, as confirmed by one of her trophies in Melee. When sweet spotted, the Lightning Kick pauses the opponent in place for a second dramatically (note that the freeze frame effect occurs in Brawl, but not in Melee), before dealing heavy electric damage and launching them away with very powerful horizontal knockback. A sourspotted hit on the other hand, delivers very low horizontal knockback with no special effects and much less damage.
Between Melee and Brawl, the attack was slightly buffed in terms of knockback when sweetspotted, along with having the aforementioned freeze frames added, though the sourspotted kick was weakened further, to where it deals less than half the damage it did in Melee, with 4% compared to 10%. The sweetspot in Brawl is more difficult to land, only lasting for 1 frame as opposed to 4 in Melee, and being about a third of the size.
In both Melee and Brawl, a sweetspotted Lightning Kick is the strongest forward and back aerial, being among the few aerial attacks that can reliably KO under 100%. Along with this great power, both forward and back aerial Lightning Kicks are fairly fast in start-up lag, while having good reach. However, in Melee both Lightning Kicks can be difficult to sweetspot with Zelda's low mobility along with the spacing requirement to avoid the flub hitboxes, which limits the usefulness of the aerials. In Brawl, the smaller window of opportunity to land the sweetspots leads to even more difficulty in landing them.
The forward and back aerial versions are very similar appearance wise and statistically. However the back aerial Lightning Kick is slightly better due to slightly faster start-up lag and slightly stronger knockback. Brawl also adds electric properties to a sweetspotted down air, making it a "Lightning Stomp" of sorts.
The Lightning Kick is very similar to Captain Falcon's forward aerial, the Knee Smash, as both are effective aerials that deal very powerful horizontal knockback with electric damage when sweetspotted, while being exceedingly weak with no special effects when sourspotted. However, the Lightning Kick hasn't reached the same level of notoriety as the Knee Smash, where the latter was more effective and prolific in competitive play for Melee. The main reason for this was that the knee was notably easier to sweetspot, as sweetspotting it only required landing the initial hitbox as opposed to landing a specifically spaced hitbox in the attack, and combined with Falcon's high mobility and great combo ability, it has seen much more utilization. Zelda's poor tournament viability and general lack of internet notoriety compared to Captain Falcon also hindered the Lightning Kick from receiving as much attention as the Knee Smash.
 See also