any process in which electrons are added to an atom or ion (as by removing oxygen or adding hydrogen); always occurs accompanied by oxidation of the reducing agent
From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
(Or "contraction") The process of transforming an expression according to certain reduction rules. The most important forms are beta reduction (application of a lambda abstraction to one or more argument expressions) and delta reduction (application of a mathematical function to the required number of arguments).
An evaluation strategy (or reduction strategy), determines which part of an expression (which redex) to reduce first. There are many such strategies.