A Poisson manifold may be thought of as a Poisson Lie algebroid, a Lie algebroid with extra structure: called an n-symplectic manifold for $n = 1$.
By Lie integration this Lie algebroid should integrate to a Lie groupoid with extra structure. Symplectic groupoids are supposed to be these objects that integrate n-symplectic manifold aka Poisson manifolds in this sense.
The groupoid algebra of these symplectic groupoids are C-star algebras that may be regarded as the quantization of the original Poisson manifold. This is described in the references below.
The original definition of (Weinstein) is this:
A symplectic Lie groupoid is a Lie groupoid $\mathbf{X}_\bullet$ whose manifold of morphisms $\mathbf{X}_1$ is equipped with a symplectic structure whose symplectic form $\omega \in \Omega^2_{closed}(\mathbf{X}_1)$ is multiplicative in that the alternating sum of its canonical pullbacks to the space $\mathbf{X}_2$ of composable morphisms vanishes:
The manifold of objects $\mathbf{X}_0$ of a symplectic Lie groupoid $\mathbf{X}_\bullet$, def. 1, carries the structure of a Poisson manifold which is unique, up to isomorphism, with the property that the target map $t \colon \mathbf{X}_1 \to \mathbf{X}_0$ is a homomorphism of Poisson manifolds (canonically regarding the symplectic manifold $(\mathbf{X}_1, \omega)$ as a Poisson manifold).
The Poisson manifolds that arise this way as $\mathbf{X}_0$ of a symplectic Lie groupoid are called integrable Poisson manifolds.
Reformulated more abstractly, def. 1 says that the differential form $\omega$, when extended to a triple
is a cocycle of degree 3 in the de Rham complex of $\mathbf{X}$, identified with the simplicial de Rham complex of the nerve $X_\bullet$ of $X$.
This observation leads to the following generalization
A pre-quasi symplectic groupoid is a Lie groupoid $\mathbf{X}$ equipped with a differential 2-form $\omega_2 \in \Omega^2(\mathbf{X}_1)$ and a differential 3-form $\omega_3 \in \Omega^3(\mathbf{X}_0)$ such that
is a cocycle in the simplicial de Rham complex of $\mathbf{X}_\bullet$, hence such that
where $\delta = \sum_{k} (-1)^k \partial_k^*$ is the alternating sum of the pullbacks along the face maps of the nerve $\mathbf{X}_\bullet$.
This appears as (Xu, def. 2.1, LG-Xu, def. 2.1). This structure is called a twisted presymplectic groupoid in (BCWZ, def. 2.1).
Since therefore a (pre-)symplectic groupoid is really a Lie groupoid equipped with a cocycle in degree-3 de Rham cohomology (instead of degree 2 as for a symplectic manifold), it is really rather an object in 2-plectic geometry.
Every Lie groupoid integrating a Poisson Lie algebroid is naturally a symplectic Lie groupoid. Picking always the unique source-simply connected integrating Lie groupoid produces a functor
When the Poisson manifold we start with happens to be a symplectic manifold, then its symplectic Lie groupoid is always the fundamental groupoid of $X$:
When $X$ is simply connected such that $\Pi(X)$ is the codiscrete groupoid $Pair(X)$ we have that the symplectic form on $Mor(\Pi(X)) = X \times X$ is $\omega \otimes (-\omega)$, for $\omega$ the symplectic form on $X$.
Conversely, for every symplectic groupoid $\mathbf{X}$ there is a unique Poisson manifold structure on its manifold $\mathbf{X}_0$ of objects such that the codomain map $t \colon \mathbf{X}_1 \to \mathbf{X}_0$ is a homomorphism of Poisson manifolds. (For instance Racaniere, theorem 6.3) One says also that $\mathbf{X}$ integrates the Poisson manifold $\mathbf{X}_0$.
The source map of a symplectic groupoid over a Poisson manifold constitutes a symplectic realization of this Poisson manifold, hence its canonical desingularization via Lie integration. See at symplectic realization for more.
In the groupoid approach to quantization symplectic groupoids are used to discuss geometric quantization not just of symplectic manifolds but more generally of Poisson manifolds.
See geometric quantization of symplectic groupoids.
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The symplectic groupoid of a Poisson manifold is also the reduced phase space of the open sector of the correspondng Poisson sigma-model. (Cattaneo-Felder 01)
Let $G$ be a Lie group with Lie algebra $\mathfrak{g}$ and consider the dual vector space $\mathfrak{g}^*$ equipped with its Lie-Poisson structure. Then the action groupoid $\mathfrak{g}^* //G$ of the coadjoint action carries a multiplicative symplectic form $\omega$ induced by the identification of the manifold of morphisms with the cotangent bundle of the group, $G \times \mathfrak{g}^* \simeq T^* G$, induced by right translation from the Poincare form on the cotangent bundle. This makes $(\mathfrak{g}^* //G, \omega)$ a symplectic groupoid which Lie integrates the Lie-Poisson structure on $\mathfrak{g}^*$.
This appears as (Bursztyn-Crainic, example 4.3).
∞-Chern-Simons theory from binary and non-degenerate invariant polynomial
(adapted from Ševera 00)
The notion of symplectic groupoids was apparently proposed independently by Karasëv, Weinstein, and Zakrzewski, all motiviated from the problem of quantization.
Alan Weinstein, Symplectic groupoids, geometric quantization, and irrational rotation algebras in Symplectic geometry, groupoids, and integrable systems (Berkeley, CA, 1989), 281–290, Springer, New York, (1991) MR1104934.
Alan Weinstein, Tangential deformation quantization and polarized symplectic groupoids, in Deformation theory and symplectic geometry (Ascona, 1996), 301–314, Kluwer (1997) MR1480730
See also the references at geometric quantization of symplectic groupoids .
Lecture notes include
The notion of pre-quasi-symplectic groupoids is introduced and the intepretation of symplectic groupoids in higher geometry is made fairly explicit in
These “pre-quasi-symplectic groupoids” had been called “twisted presymplectic groupoids” in
The identification with reduced phase spaces of the open Poisson sigma-model is in
Further developments include
Ping Xu, Morita equivalence and symplectic realizations of Poisson manifolds, Annales scientifiques de l’École Normale Supérieure, Sér. 4, 25 no. 3 (1992) (NUMDAM)
Henrique Bursztyn, Marius Crainic, Dirac structures, momentum maps and quasi-Poisson manifolds (pdf)
The formal groupoid version of symplectic groupoids is discussed in