Ice Slasher

Ice Slasher (アイススラッシャー) is Ice Man's Special Weapon. It takes the form of an arrowhead-shaped blade of ice that can freeze enemies with a temperature of 200 degrees below zero.

Mega Man
Mega Man receives Ice Slasher after defeating Ice Man. Only one Ice Slasher can be present in the screen at a time, and it passes through most obstacles. This is Fire Man's weakness, and except for him and the first form of Wily Machine 1, all bosses are immune to it.

Ice Slasher doesn't cause any damage to enemies, but it can be used to temporarily stop them, allowing Mega Man to switch to another weapon and attack frozen enemies. It is specially useful to stop enemies like Flying Shell, Sniper Joe and Big Eye to safely pass by them. When the flame geysers that appear in Fire Man's stage and in the first stage of Dr. Wily's Robot Manufacturing Plant are frozen, they can be used as steps.

Mega Man: Powered Up
Ice Slasher now can cause harm to all bosses due to Ice Man being a playable character, and it can do a very small amount of damage to enemies. Ice Man can use frozen enemies as platforms to reach higher areas.

Mega Man: Dr. Wily's Revenge
Like its appearance in the first Mega Man game, Ice Slasher is obtained from Ice Man and is Fire Man's weakness. Flash Man is also weak to this weapon. Cut Man, Elec Man, Heat Man, Enker, and Wily Machine I are immune to it.

Mega Man: The Wily Wars
Like all weapons from the first three games, Mega Man can use Ice Slasher in the Wily Tower mode. This is one of the weaknesses of Mega Water S. The second form of the Wily Machine is immune to it.

Mega Man: The Power Battle
Ice Slasher is obtained by defeating Ice Man in the Mega Man 1~2 course. It is Heat Man's weakness.

Street Fighter X Tekken
Bad Box Art Mega Man can use the Ice Slasher as an attack in Street Fighter X Tekken.

Super Smash Bros. for Nintendo 3DS / Wii U
Ice Slasher is used by Mega Man in Super Smash Bros. for Nintendo 3DS / Wii U as a custom move for his side special, Crash Bomber. It must be unlocked before it can be swapped out.

Mega Man fires an icy spear that only deals 4%, half the damage the Crash Bomber deals. However, it can pierce through multiple opponents with one shot, and at medium damage percentages, it may freeze them.

Mega Man
Damage values for the original Mega Man.

Mega Man: Dr. Wily's Revenge
Damage values in Megaman: Dr. Wily Revenge.

Mega Man (Ruby-Spears)
In the animated series, Ice Slasher differs from its in-game depiction in the fact that it takes the form of a beam of shatteringly cold energy capable of freezing almost anything within a few seconds of exposure. When obtained by Mega Man in the episodes The Beginning and Ice Age, he fires a freezing beam from his buster.

Ice Man can use Ice Slasher from any of his hands, usually moving his hand to the side before releasing it. When using Ice Slasher with both hands, his cupped hands are drawn to his side and then thrust forward to shoot out the streaming, freezing beam.

At one point, when Ice Man himself used it in the episode Ice Age, he used Ice Slasher to create a giant hammer of ice to throw at Mega Man. He also used it to provide the icy armor for the Ice Bots he used later to attack Doctor Wily and Air Man with after they landed in the city.

Curiously, Ice Man did not refer to his power by its actual name, but called it "freeze ray" when he called for Air Man's help after Mega Man took it from him. Dr. Light did call it by its correct name in the first episode however, being one of the few weapons to have their name mentioned in the series.

Mega Man (Archie Comics)
In Let the Games Begin!, Mega Man obtained Ice Slasher after defeating Ice Man and used it to defeat Fire Man.

Trivia

 * Ice Slasher was misspelled several times as "Icelasher" (アイスラッシャー) in early sources, including Japanese ads, the Rockman Kanzen Hisshou Bon guide, and in a question from Adventure Quiz: Capcom World 2 asking what weapon is obtained from Ice Man (the wrong answers being Ice Bomber, Ice Shooter, and Ice Candy). It was also named "Ice Crusher" (アイスクラッシャー) in the Rockman Complete Works official site.
 * The way Ice Man uses Ice Slasher in the Ruby-Spears animated series is similar to the Kamehameha in the Dragon Ball series.