what is copula?

Answer 1

"In statistics, a copula is a multivariate cumulative distribution function defined on the n-dimensional unit cube [0, 1]n such that every marginal distribution is uniform on the interval [0, 1]."

"The copula contains all of the information on the nature of the dependence between a set of random variables that can be given without the marginal distributions, but gives no information on the marginal distributions. In effect, the information on the marginals and the information on the dependence are neatly separated from each other."

http://www.answers.com/topic/sklar-s-theorem

Answer 2

A copula is an intransitivity verb.

See
http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsACopula.htm