A roll of 20 on a d20 (a twenty sided dice) before any modifiers are added. In other words, when an 20 actually appears on the dice.
In many role playing games, a natural 20 confers some benefit above and beyond the regular rules. For example, in the AD&D system a natural 20 is an automatic hit, and in many house rules doubles damage in an attack roll.
The d20 system complicated the concept slightly, when they introducted their own version of the critical hit system. Now a natural 20 doesn't guarantee double damage, and a critical hit isn't limited to a natural 20. Much of the magic persists, however, when you see the triangular face with the top possible roll. A natural 20 also has special rules in other parts of the d20 system: skills, magic, and saves, for example.
No matter how long you've been playing role playing games, it seems a natural 20 is always a source of joy and merriment
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