Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria (e.g. "two counties over"). In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, and provides a concrete way of describing what it means for elements of some space to be "close to" or "far away from" each other.
In most cases, "distance from A to B" is interchangeable with "distance between B and A".
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In neutral geometry, the distance between (x_{1}) and (x_{2}) is the length of the line segment between them:
In analytic geometry, the distance between two points of the xyplane can be found using the distance formula. The distance between (x_{1}, y_{1}) and (x_{2}, y_{2}) is given by:
Similarly, given points (x_{1}, y_{1}, z_{1}) and (x_{2}, y_{2}, z_{2}) in threespace, the distance between them is:
These formulae are easily derived by constructing a right triangle with a leg on the hypotenuse of another (with the other leg orthogonal to the plane that contains the 1st triangle) and applying the Pythagorean theorem.
In the study of complicated geometries, we call this (most common) type of distance Euclidean distance, as it is derived from the Pythagorean theorem, which does not hold in NonEuclidean geometries. This distance formula can also be expanded into the arclength formula.
In the Euclidean space R^{n}, the distance between two points is usually given by the Euclidean distance (2norm distance). Other distances, based on other norms, are sometimes used instead.
For a point (x_{1}, x_{2}, ...,x_{n}) and a point (y_{1}, y_{2}, ...,y_{n}), the Minkowski distance of order p (pnorm distance) is defined as:
1norm distance  
2norm distance  
pnorm distance  
infinity norm distance  
p need not be an integer, but it cannot be less than 1, because otherwise the triangle inequality does not hold.
The 2norm distance is the Euclidean distance, a generalization of the Pythagorean theorem to more than two coordinates. It is what would be obtained if the distance between two points were measured with a ruler: the "intuitive" idea of distance.
The 1norm distance is more colourfully called the taxicab norm or Manhattan distance, because it is the distance a car would drive in a city laid out in square blocks (if there are no oneway streets).
The infinity norm distance is also called Chebyshev distance. In 2D, it is the minimum number of moves kings require to travel between two squares on a chessboard.
The pnorm is rarely used for values of p other than 1, 2, and infinity, but see; super ellipse.
In physical space the Euclidean distance is in a way the most natural one, because in this case the length of a rigid body does not change with rotation.
The Euclidean distance between two points in space ( and ) may be written in a variational form where the distance is the minimum value of an integral:
Here is the trajectory (path) between the two points. The value of the integral (D) represents the length of this trajectory. The distance is the minimal value of this integral and is obtained when r = r ^{*} where r ^{*} is the optimal trajectory. In the familiar Euclidean case (the above integral) this optimal trajectory is simply a straight line. It is well known that the shortest path between two points is a straight line. Straight lines can formally be obtained by solving the EulerLagrange equations for the above functional. In nonEuclidean manifolds (curved spaces) where the nature of the space is represented by a metric g_{ab} the integrand has be to modified to , where Einstein summation convention has been used.
The Euclidean distance between two objects may also be generalized to the case where the objects are no longer points but are higherdimensional manifolds, such as space curves, so in addition to talking about distance between two points one can discuss concepts of distance between two strings. Since the new objects that are dealt with are extended objects (not points anymore) additional concepts such as nonextensibility, curvature constraints, and nonlocal interactions that enforce noncrossing become central to the notion of distance. The distance between the two manifolds is the scalar quantity that results from minimizing the generalized distance functional, which represents a transformation between the two manifolds:
The above double integral is the generalized distance functional between two plymer conformation. s is a spatial parameter and t is pseudotime. This means that is the polymer/string conformation at time t_{i} and is parameterized along the string length by s. Similarly is the trajectory of an infinitesimal segment of the string during transformation of the entire string from conformation to conformation . The term with cofactor λ is a Lagrange multiplier and its role is to ensure that the length of the polymer remains the same during the transformation. If two discrete polymers are inextensible, then the minimaldistance transformation between them no longer involves purely straightline motion, even on a Euclidean metric. There is a potential application of such generalized distance to the problem of protein folding^{[1]}^{[2]} This generalized distance is analogous to the NambuGoto action in string theory, however there is no exact correspondence because the Euclidean distance in 3space is inequivalent to the spacetime distance minimized for the classical relativistic string.
The algebraic distance is a metric often used in computer vision that that can be minimized by least squares estimation. [1][2] For curves or surfaces given by the equation x^{T}Cx = 0 (such as a conic in homogeneous coordinates), the algebraic distance from the point x' to the curve is simply x'^{T}Cx'. It may serve as an "initial guess" for geometric distance to refine estimations of the curve by more accurate methods, such as nonlinear least squares.
In mathematics, in particular geometry, a distance function on a given set M is a function d: M×M → R, where R denotes the set of real numbers, that satisfies the following conditions:
Such a distance function is known as a metric. Together with the set, it makes up a metric space.
For example, the usual definition of distance between two real numbers x and y is: d(x,y) = x − y. This definition satisfies the three conditions above, and corresponds to the standard topology of the real line. But distance on a given set is a definitional choice. Another possible choice is to define: d(x,y) = 0 if x = y, and 1 otherwise. This also defines a metric, but gives a completely different topology, the "discrete topology"; with this definition numbers cannot be arbitrarily close.
Various distance definitions are possible between objects. For example, between celestial bodies one should not confuse the surfacetosurface distance and the centertocenter distance. If the former is much less than the latter, as for a LEO, the first tends to be quoted (altitude), otherwise, e.g. for the EarthMoon distance, the latter.
There are two common definitions for the distance between two nonempty subsets of a given set:
The distance between a point and a set is the infimum of the distances between the point and those in the set. This corresponds to the distance, according to the firstmentioned definition above of the distance between sets, from the set containing only this point to the other set.
In terms of this, the definition of the Hausdorff distance can be simplified: it is the larger of two values, one being the supremum, for a point ranging over one set, of the distance between the point and the set, and the other value being likewise defined but with the roles of the two sets swapped.
In graph theory the distance between two vertices is the length of the shortest path between those vertices.
Distance cannot be negative. Distance is a scalar quantity, containing only a magnitude, whereas a displacement vector is a vector quantity characterized by both magnitude and direction.
The distance covered by a vehicle (often recorded by an odometer), person, animal, or object along a curved path from a point A to a point B should be distinguished from the respective displacement (the distance along a straight line from A to B). For instance, the distance covered during a round trip from A to B and back to A may be very long, while the displacement is always zero (because starting and ending points coincide).
Directed distances are distances with a direction or sense. They can be determined along straight lines and along curved lines. A directed distance along a straight line from A to B is a vector joining any two points in a ndimensional Euclidean vector space. A directed distance along a curved line is not a vector and is represented by a segment of that curved line defined by endpoints A and B, with some specific information indicating the sense (or direction) of an ideal or real motion from one endpoint of the segment to the other (see figure). For instance, just labelling the two endpoints as A and B can indicate the sense, if the ordered sequence (A, B) is assumed, which implies that A is the starting point.
A displacement (see above) is a special kind of directed distance defined in mechanics. A directed distance is called displacement when it is the distance along a straight line (minimum distance) from A and B, and when A and B are positions occupied by the same particle at two different instants of time. This implies motion of the particle.
Another kind of directed distance is that between two different particles or point masses at a given time. For instance, the distance from the center of gravity of the Earth A and the center of gravity of the Moon B (which does not strictly imply motion from A to B).
Circular distance is the distance traveled by a wheel. The circumference of the wheel is 2*(pi)*(radius), and assuming the radius to be 1, then each revolution of the wheel is equivalent of the distance 2*(pi) radians. In engineering (omega)=2*(pi)*f is used a lot, where f is the frequency.

Distance is the quality of objects being physically separated from one another.
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Distance is how far one thing is from another thing. Distance is a measure of the space between two things.
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Distance is different from displacement. Displacement is the difference between a starting point and a finishing point.
A and B are 1 metre apart. Tom walks from A to B. Then Tom walks from B back to A. The distance Tom walked is 2 metres. Tom's displacement is 0 metres, because Tom started at A and finished at A.
