A critical hit is a hit that "bypasses" the enemy DFP.

Some weapons enhance you critical hit rate. Moreover, most of these weapons multiply the critical hit DMG! 
Here's a list (sorted by critical hit rate and DMG multiplier):

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5 % and crit. DMG * 2:
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Aqua sword
Boom sword
Burn sword
Freeze sword
Super sword
Flying ax

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8 % and crit. DMG * 3:
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Deadly sword (uncursed)
  Description: The power of "Deadly sword" turned into an actual sword.

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10 % and crit. DMG * 1:
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Gades blade

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10 % and crit. DMG * 2:
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Blaze sword
Dekar blade
Dual blade 
Fry sword 
Lizard blow
Sizzle sword
Sky sword
Snow sword
Flying blow
Mega ax
Dragon spear
Water spear
Spark staff
Air whip
Holy whip

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25 % and crit. DMG * 3:
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Dragon blade

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40 % and crit. DMG * 1:
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Luck rapier (uncursed)

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40 % and crit. DMG * 1,25:
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Lucky blade (uncursed)

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50 % and crit. DMG * 0,125 (=> DMG / 8 !!!):
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Fortune whip

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50 % and crit. DMG * 1:
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Beserk blade (uncursed)

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50 % and crit. DMG * 3:
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Deadly sword (cursed)
  Description: The power of "Deadly sword" turned into an actual sword.

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100 % and crit. DMG * 1:
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Luck rapier (cursed)

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100 % and crit. DMG * 1,25:
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Lucky blade (cursed)

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