A meteor smash is an attack in in the Super Smash Bros. series and Super Smash Flash series that possesses a very low knockback angle, hitting the opponent almost straight down on contact. In most cases, it can be meteor cancelled by jumping or by using a recovery move. If it can not be canceled, then it is a spike which sends the opponent downwards until the hitstun wears off.
In Super Smash Flash
According to the How to play tutorial, meteor smashes are present in Super Smash Flash. This, however, is something really difficult to perform as the physics are very different to the official Super Smash Bros. games and some are not a certainty. More often than not, the player initiating the move would risk dying themselves. However, with good timing, some of the down aerials, such as Lloyd's Rising Falcon or Fox's Fire Fox, were guaranteed to KO most opponents at very low percentages.
In Super Smash Flash 2
The physics of Super Smash Flash 2 are changed to make them similar to the Super Smash Bros. games. Meteor smashes are easy to perform on this game but still require good timing to be performed correctly. The information on the section below is subject to change.
Turns to the side, extends his arms behind himself, and flips forward while swinging them. This meteor smash is more difficult to use than Donkey Kong's down aerial because it requires very specific timing.
This is a stall-then-fall. Super Saiyan Goku dashes downwards while upside down, body engulfed in yellow energy. It is a powerful meteor smash that deals one hit and goes through opponents, and since Super Saiyan Goku can jump infinitely and use his up special infinitely, this move is safe to use off-stage.
This is a stall-then-fall. In the air, Ichigo thrusts and holds his blade horizontally underneath himself before falling quickly. In the grounded version, he does a flash step upwards before falling down. This move will leave Ichigo unable to recover because he will still be in the move's animation until he touches the ground or grabs a ledge (although the move does not cause helplessness) and will make him self-destruct if the move is done offstage. He is able to move horizontally slightly.
Performs a downwards drill kick. It hits multiple times, though each hit is quite weak. All hits can meteor smash. Despite the move being extremely weak, Kirby can utilize it to drag the opponent down with him for an easy KO, and he can easily recover, due to his great vertical recovery.
Wields a sword, jumps up into the air, and falls down quickly with his sword horizontally held in front of him. If it hits a character in the air while falling down, it heavily meteor smashes them downwards.
Pressing down on the third slash produces an extremely weak meteor smash hitbox. If opponents touch the tip of the sword, they are sent sideways instead. Pressing up on the third slash produces a meteor smash.
Slashes his sword downwards, anybody hit with the tip will be powerfully meteor smashed.
Kicks downwards, stomping the opponent. If it connects with her foot upon the first frame of execution, the opponent is briefly stunned and rockets downwards. If it connects after the first frame or anywhere else, it produces a weak meteor smash hitbox.
Performs a flip jump, and, if an attack button is pressed during her jump, she will stick out her foot. Anyone comes into contact with her leg or foot during the first few frames of the attack will be meteor smashed. If the move connects after the first few frames, the opponent will be sent upwards.