In mathematics, a chiral algebra is an algebraic structure introduced by Beilinson and Drinfeld (2004) as a rigorous version of the rather vague concept of a chiral algebra in physics. In Chiral Algebras, Beilinson and Drinfeld introduced the notion of chiral algebra, which based on the pseudo-tensor category of D-modules. They give a 'coordinate independent' notion of vertex algebras, which are based on formal power series. Chiral algebras on curves are essentially conformal vertex algebras.

A chiral algebra on a smooth algebraic curve ${\displaystyle X}$ is a right D-module ${\displaystyle {\mathcal {A}}}$, equipped with a D-module homomorphism $${\displaystyle \mu :{\mathcal {A}}\boxtimes {\mathcal {A}}(\infty \Delta )\rightarrow \Delta _{!}{\mathcal {A}}}$$ on ${\displaystyle X^{2}}$ and with an embedding ${\displaystyle \Omega \hookrightarrow {\mathcal {A}}}$, satisfying the following conditions

$${\displaystyle {\begin{array}{lcl}&\Omega \boxtimes {\mathcal {A}}(\infty \Delta )&\rightarrow &{\mathcal {A}}\boxtimes {\mathcal {A}}(\infty \Delta )&\\&\downarrow &&\downarrow \\&\Delta _{!}{\mathcal {A}}&\rightarrow &\Delta _{!}{\mathcal {A}}&\\\end{array}}}$$ Where, for sheaves ${\displaystyle {\mathcal {M}},{\mathcal {N}}}$ on ${\displaystyle X}$, the sheaf ${\displaystyle {\mathcal {M}}\boxtimes {\mathcal {N}}(\infty \Delta )}$ is the sheaf on ${\displaystyle X^{2}}$ whose sections are sections of the external tensor product ${\displaystyle {\mathcal {M}}\boxtimes {\mathcal {N}}}$ with arbitrary poles on the diagonal: $${\displaystyle {\mathcal {M}}\boxtimes {\mathcal {N}}(\infty \Delta )=\varinjlim {\mathcal {M}}\boxtimes {\mathcal {N}}(n\Delta ),}$$ ${\displaystyle \Omega }$ is the canonical bundle, and the 'diagonal extension by delta-functions' ${\displaystyle \Delta _{!}}$ is $${\displaystyle \Delta _{!}{\mathcal {M}}={\frac {\Omega \boxtimes {\mathcal {M}}(\infty \Delta )}{\Omega \boxtimes {\mathcal {M}}}}.}$$

The category of vertex algebras as defined by Borcherds or Kac is equivalent to the category of chiral algebras on ${\displaystyle X=\mathbb {A} ^{1}}$ equivariant with respect to the group ${\displaystyle T}$ of translations.

Chiral algebras can also be reformulated as factorization algebras.