Duplication: Part of a chromosome in duplicate, a particular kind of mutation (change) involving the production of one or more copies of any piece of DNA, including a gene or even an entire chromosome.
Duplications typically arise from an event termed unequal crossing-over (a kind of recombination) that occurs between misaligned homologous chromosomes during meiosis (germ cell formation). The chance of this event happening is a function of the degree of sharing of repetitive elements between two chromosomes. The recombination products of such an event are a duplication at the site of the exchange and a reciprocal deletion. A remarkable class of duplications in which the duplicated region has popped up far away from home base has also been discovered.
A duplication is the opposite of a deletion.