A critical hit, also called a critical success, critical win or simply a crit, is an especially successful action that has an enhanced effect, usually as a result of an unlikely and positive dice roll (or equivalent in other forms of random resolution). The term critical hit is the most common, but generally only refers to cases when the attempted action is an attack, in which case the enhanced effect is usually to deal extra damage (but could also result in a condition being applied).
Critical hits are a simple way of using degrees of success.
The 1975 role-playing game Empire of the Petal Throne introduced the concept of critical hits (though not the phrase). Using these rules, a player who rolls a 20 on a 20-sided die does double the normal damage, and a 20 followed by a 19 or 20 counts as a killing blow. According to creator M.A.R. Barker, "this simulates the 'lucky hit' on a vital organ."
Critical hits are meant to simulate an occasional "lucky hit". The concept represents the effect of hitting an artery, or finding a weak point, such as a stab merely in the leg causing less damage than a stab in the Achilles tendon. Critical hits are almost always random, although character attributes or situational modifiers may come into play. For example, games in which the player characters have a "Luck" attribute will often base the likelihood of critical hits occurring on this statistic: a character with high Luck will deal a higher percentage of critical hits, while a character with low Luck may, in some games, be struck by more critical hits. In the role playing game Dungeons & Dragons, when a player character attacks an opponent the player typically rolls a 20-sided die; a roll of 20 (a 5% chance) results in a critical hit.
The most common kind of critical hit simply deals additional damage, most commonly dealing double the normal damage that would have been dealt, but many other formulas exist as well (such as ignoring defense of the target or always awarding the maximum possible damage). Critical hits also occasionally do "special damage" to represent the effects of specific wounds (for example, losing use of an arm or eye, or being reduced to a limp). Critical hits usually occur only with normal weapon attacks, not with magic or other special abilities, but this depends on the individual game's rules.
Many tabletop games use hit point systems in which wounded characters often have no game differences from unwounded characters other than a reduction in hit points. Critical hits originally provided a way to simulate wounds to a specific part of the body. These systems usually use lookup charts and other mechanics to determine which wound was inflicted. In RPGs with non-humanoid characters or monsters, unlikely or bizarre results could occur, such as a Beholder with a "lost leg". Most systems now simply award extra damage on a critical hit, trading realism for ease of play. The effect of a critical hit is to break up the monotony of a battle with high, unusual results.
The roleplaying game Rolemaster is known for its extended system of criticals. One long-standing claim from its company ICE is that it is not the normal hits that kill, but the critical. By integrating criticals even on low results by varying the critical severity (from A (minor) - J (extreme)) and the large variety of criticals (e.g. Slash, Krush, Puncture, Heat, Cold, Electricity, Impact, Unarmed Strikes and even some bizarre ones such as Internal Disruption and Essence criticals) every combat plays out differently. Critical results vary from simple additional hits, and added bleeding and stuns to limbs lopped off and internal organs destroyed. Player characters are not immune to the effects of a critical hit in this system.
The negative counterpart of the critical hit is variously known as the critical miss, critical failure, botch or fumble. The concept is less frequently borrowed than that of critical hits.