In mathematics

• Multiplicative inverse
  Multiplicative inverse
  In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
  
  , in mathematics, the number 1/x, which multiplied by x gives the product 1, also known as a reciprocal
• Reciprocal rule
  Reciprocal rule
  In calculus, the reciprocal rule is a shorthand method of finding the derivative of a function that is the reciprocal of a differentiable function, without using the quotient rule or chain rule....
  
  , a technique in calculus for calculating derivatives of reciprocal functions
• Reciprocal spiral
  Hyperbolic spiral
  thumb|200px|right|Hyperbolic spiral for a=2A hyperbolic spiral is a transcendental plane curve also known as a reciprocal spiral. A hyperbolic spiral is the opposite of an Archimedean spiral and are a type of Cotes' spiral.It has the polar equation:...
  
  : a mathematical function
• Reciprocal polynomial, in mathematics

In science and technology

• Reciprocal length
  Reciprocal length
  Reciprocal length or inverse length is a measurement used in several branches of science and mathematics. As the reciprocal of length, common units used for this measurement include the reciprocal metre or inverse metre , the reciprocal centimetre or inverse centimetre , and, in optics, the...
  
  , a measurement used in science
• Reciprocal lattice
  Reciprocal lattice
  In physics, the reciprocal lattice of a lattice is the lattice in which the Fourier transform of the spatial function of the original lattice is represented. This space is also known as momentum space or less commonly k-space, due to the relationship between the Pontryagin duals momentum and...
  
  , a basis for the dual space of covectors, in crystallography
• Reciprocating oscillation
  Reciprocating oscillation
  Reciprocating oscillation is an action of oscillation where the oscillation vector varies its amplitude, frequency and or displacement reciprocally....
  
   in physical wave theory
• Reciprocal system as a physical theory
• Reciprocating engine
  Reciprocating engine
  A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
  
   or piston engine
• Reciprocal aircraft heading, 180 degrees (the opposite direction) from a stated heading

Life sciences and medicine

• Hybrid (biology), in genetics, the result of reciprocal crossing, also known as a reciprocal hybrid
• Reciprocal cross
  Reciprocal cross
  In genetics, a reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. All parent organisms must be true breeding to properly carry out such an experiment. In one cross, a male expressing the trait of interest will be crossed with a...
  
  , a breeding experiment in genetics
• Reciprocal altruism
  Reciprocal altruism
  In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time...
  
  , a form of symbiotic relationship in evolutionary biology
• Sherrington's law of reciprocal innervation
  Sherrington's law of reciprocal innervation
  Sherrington's law of reciprocal innervation, also called Sherrington's law II explains how a muscle will relax when its opposite muscle is activated. René Descartes had hypothesized as much in 1626...
  
   in the theory of muscle activation

In reciprocal relationships

• Reciprocal link between two web pages
• Reciprocal license, a type of software licenses also known as copyleft
• Reciprocal Public License
  Reciprocal Public License
  The Reciprocal Public License is a software license inspired by the GNU General Public License but authored to explicitly remove what some have referred to as the GPL's "privacy loophole"—an aspect of the GPL which allows recipients of GPL'd code to:#make changes to source code which are...
  
  (RPL), a software license