What is "quasi-bialgebra"

In mathematics, quasi-bialgebras are a generalization of bialgebras: they were first defined by the Ukrainian mathematician Vladimir Drinfeld in 1990. A quasi-bialgebra differs from a bialgebra by having coassociativity replaced by an invertible element Φ which controls the non- coassociativity. One of their key properties is that the corresponding category of modules forms a tensor category.