Reduction

Reduction in a general sense means to take of a part of a whole thing or to make it smaller. 

'Reduction or reducing may may mean in special

 Reduction (chemistry), the reverse of oxidation, half of a redox reaction, adding of electrons  to a molecule, which means reducing the valence
 Reduction (cooking), the process of thickening a liquid mixture such as a sauce by evaporation
 Reduction (philosophy), in philosophy, the process of explaining a particular concept or phenomenon in terms of more fundamental concepts or phenomena
 Reduction (linguistics), in linguistics, the shortening of the pronunciation of a word. e.g. wanna, lemme, gimme.
 Reduction, in physiology, another name for weight loss or dieting
 Reduction (orthopedic surgery), in orthopedic surgery, a medical procedure to restore a fracture to a correct alignment
 Reduction (town), a form of Catholic mission in South America in the 17th and 18th centuries
 Purchasing reduction, in economics and in waste management, is the process of decreasing the purchase of consumer goods
 Reduction (Sweden), in 1680 a return of lands to the Crown earlier granted to the nobility.
 Waste reduction is the first and most desirable component of the waste hierarchy (reduce, reuse, recycle)

In mathematics and computer science''':
 Reduction (mathematics), the process of manipulating a series of equations or matrices into a desired 'simpler' format
 Reduction property, in descriptive set theory, a pointclass allows partitioning the union of two sets in the pointclass into two disjoint sets in the same pointclass
 Reduction (complexity), in computational complexity theory, the transformation of an instance of one problem into an instance of another
 Reduce computer algebra system, a general-purpose computer algebra system geared towards applications in physics.
 Reduce (higher-order function), a family of higher-order functions that process a data structure in some order and build up a return value.