Circular error probability
Circular error probability is the most common metric of the accuracy to which a weapon can be delivered. It is the radius of a circle within which half of a missile’s projectiles are expected to fall.
Within the circle in which the missiles fall, the dispersion is the variation, around the mean point of impact of projectiles delivered as consistent a manner as possible (e.g., from a fixed launcher to a fixed target). With some weapons intended for area rather than point coverage, such as unguided rockets fired from a multiple rocket launcher or cluster submunitions, dispersion, as long it is within the area where weapon effects are intended to occur, greater dispersion is actually an advantage.
In contrast with weapons that deliver multiple projectiles over an error, delivery error is relevant for weapons that are used against point targets. It describes the statistical distribution of weapons on and near the point target. As long as the radius of effect of the weapon includes the target point, the delivery error may be insignificant. Obviously, the radius of effect of a nuclear weapon and a pistol bullet will be quite different; delivery error has to consider both the weapon's effect and the accuracy of its delivery. Deviation is a similar metric, but applied to an individual projectile's miss distance rather than the distribution of a set of projectiles.
Horizontal error describes the error, in direction as well as distance, by which a projectile is as likely to miss, as well as hit, the target. The shape of the horizontal error will vary with the type of weapons delivery; a bomb dropped vertically from a helicopter hovering while there is no wind will have a circular dispersion pattern. The dispersion pattern of a projectile fired from a gun, or dropped from a moving aircraft, will tend to be elliptical, in the direction of motion of the projectile.