symmetric monoidal (∞,1)-category of spectra
Let $\mathbf{k}$ be a field of characteristic 0 and $\lambda \in \mathbf{k}^*$. A $\lambda$-Drinfeld associator, or just $\lambda$-associator, is a grouplike element $\Phi(a,b)$ of the $\mathbf{k}$-algebra of formal power series in two non-commuting variables $a,b$ satisfying:
in $\widehat{U(L_4)}$
where $L_4$ is the fourth Drinfeld-Kohno Lie algebra and $c=-a-b$.
The set of “0-associators” is the what is called the Grothendieck-Teichmueller group. This acts freely on the set of Drinfeld associators.
These equations are modelled on the defining axioms of braided monoidal categories. Indeed, associators provides a universal way of constructing braided monoidal categories out of some Lie algebraic data.
Drinfeld associators are also used to construct quasi-Hopf algebras.
Let $(\mathfrak{g},t)$ be a metrizable Lie algebra, that is a Lie algebra $\mathfrak{g}$ together with a non-degenerate symmetric $\mathfrak{g}$-invariant 2-tensor $t$. Then if $\Phi$ is a $\lambda$-associator and $\hbar$ a formal variable, then the action of
and $e^{\hbar \lambda t/2}\circ P$ turns the category of $U(\mathfrak{g} ) [ [ \hbar ] ]$ module into a braided monoidal category, where $P$ is the flip: $P(a\otimes b)=b\otimes a$.
Examples of metrizable Lie algebras are provided by simple Lie algebras, in which case $t$ is a scalar mutliple of the Killing form. The braided monoidal category obtained this way is equivalent to that constructed from the corresponding quantum group, by the Drinfeld-Kohno theorem.
An explicit associator over $\mathbf{C}$ was constructed by Drinfeld from the monodromy of a universal version of the Knizhnik-Zamolodchikov equation. Using the non-emptiness of the set of associators, and the fact that is is a torsor under the action of the Grothendieck-Teichmueller group, he show that associators over $\mathbf{Q}$ also exists.