The term "d20" is most often used to describe a twenty-sided die, or the roll of such a die. The die contains numbers from one to 20, and each result has an equal probability of appearing (5%). Such a die is used in various role playing games, most notably Dungeons and Dragons. The term can also be used to describe the d20 Game System from Wizards of the Coast, which focuses heavily on twenty-sided dice.
The twenty-sided die can be used in many ways, but most typically, a roll will be made to determine success or failure for a specific challenge; for instance, attempting to hit an opponent in combat, or avoid a special effect (known in D&D as a saving throw). Often, such a roll reduces a complex system with multiple dependencies down to a discrete, success-or-failure result, in order to keep such systems manageable: in D&D, an attacker's strength, skill with his weapon, his weapon's properties, the target's agility, the target's physical defenses (armor and shield or a thick hide, for instance), along with all magical and environmental intervening factors, are reduced to a single "hit" or "miss" result. Dungeons & Dragons: Tiny Adventures uses this all-or-nothing die roll approach.