symmetric monoidal (∞,1)-category of spectra
Given a Lie algebra $L$ internal to some symmetric monoidal $k$-linear category $C = (C,\otimes, \mathbf{1},\tau)$, an enveloping monoid (or enveloping algebra) of $L$ in $C$ is any morphism $f: L\to Lie(A)$ of Lie algebras in $C$ where $A$ is a monoid (= algebra) in $C$, and $Lie(A)$ is the underlying object of $A$ equipped with the Lie bracket $[,]_{Lie(A)}=\mu-\mu\circ\tau_{A,A}$. In further we will just write $A$ for $Lie(A)$. A morphism of enveloping algebras $\phi : (f:L\to A)\to (f':L\to A')$ is a morphism $g: A\to A'$ of monoids completing a commutative triangle of morphisms in $C$, i.e. $g\circ f = f'$. With an obvious composition of morphisms, the enveloping algebras of $L$ form a category. A universal enveloping algebra of $L$ in $C$ is any universal initial object $i_L:L\to U(L)$ in the category of enveloping algebras of $L$; it is of course unique up to an isomorphism if it exists. If it exists for all Lie algebras in $C$, then the rule $L\mapsto U(L)$ can be extended to a functor $U$ which is the left adjoint to the functor $Lie:A\mapsto Lie(A)$ defined above and the morphism $i_L:L\to U(L)$ is the unit of the adjunction.
In the more general context of higher algebra there is a notion of universal enveloping E-n algebra of an L-infinity algebra for all $n \in \mathbb{N}$ which generalizes the notion of universal associative algebra envelope of a Lie algebra. See at universal enveloping E-n algebra.
The existence of the universal enveloping algebra is easy in many concrete symmetric monoidal categories, e.g. the symmetric monoidal category of bounded chain complexes (giving the universal enveloping dg-algebra of a dg-Lie algebra), but not true in general.
First of all if $C$ admits countable coproducts, form the tensor algebra $TL=\coprod_{n=0}^\infty L^{\otimes n}$ on the object $L$; this is a monoid in $C$. In most standard cases, one can also form the smallest 2-sided ideal (i.e. $A$-subbimodule) $I$ in monoid $A$ among those ideals whose inclusion into $A$ is factorizing the map $([,]-m_{TL}+m_{TL}\circ\tau)\circ \otimes :L\otimes L\to TL$; if the coequalizers exist in $C$ then we can form the quotient object $TL/I$ and there is an induced monoid structure in it. Under mild conditions on $C$, the natural morphism $i_L:L\to TL/I$ is an universal enveloping monoid of $L$ in $C$. If $C$ is an abelian tensor category and some flatness conditions on the tensor product are satisfied, then the enveloping monoid $i_L:L\to TL/I$ is a monic morphism in $C$ and $U(L\coprod L)\cong U(L)\otimes U(L)$.
The isomorphism problem for enveloping algebras is about the fact that the universal enveloping monoids of two Lie algebras of $C$ are isomorphic as associative monoids in $C$, but this does not imply that the Lie algebras are isomorphic. This is even not true in general for the Lie $k$-algebras (in classical sense), even if $k$ is a field of characteristics zero. It is known however in that case that the dimension of the finite-dimensional Lie $k$-algebra $L$ can be read off from its universal enveloping $k$-algebra as its Gel’fand-Kirillov dimension $GK(U(L))$.
The universal enveloping algebra $U(\mathfrak{g})$ of a Lie algebra is naturally a (non-commutative) Poisson algebra with the restriction of the Poisson bracket to generators being the original Lie bracket
Suppose the universal enveloping algebras of Lie algebras exist in a $k$-linear symmetric monoidal category $C$ and the functorial choice $L\mapsto U(L)$ realizing the above construction with tensor products is fixed. For example, this is true in the category of $k$-modules where $k$ is a commutative ring. Then the projection $L\to 0$ (where $0$ is the trivial Lie algebra) induces the counit $\epsilon:U(L)\to U(0)=\mathbf{1}$. The coproduct $\Delta:U(L)\to U(L\times L)\cong U(L)\otimes U(L)$ is induced by the diagonal map $L\to L\times L$ whereas the antipode $S=U(-id):U(L)\to U(L)$. One checks that these morphisms make $U(L)$ into a Hopf algebra in $C$. (e.g Milnor-Moore 65, section 5) The Milnor-Moore theorem states conditions under which the converse holds (hence under which a primitively generated Hopf algebra is a universal enveloping algebra of a Lie algebra).
If the category is simply the vector spaces over a field $k$, then for $l\in L$, after we identify $L$ with its image in $U(L)$, $\Delta(l) = l\otimes 1 + 1\otimes l$, i.e. the elements in $L$ are the primitive elements in $U(L)$.
The Poincaré–Birkhoff–Witt theorem states that the associated graded algebra of an enveloping algebra $U(g)$ in characteristics zero is canonically isomorphic to a symmetric algebra $Sym(g)$, and $U(g)$ is isomorphic to $S(g)$ as a coalgebra, via the projection map $U(g)\to Gr U(g)$.
The universal enveloping algebra of the tangent Lie algebra of a finite-dimensional Lie group $G$ over real or complex numbers is canonically isomorphic to the algebra of the left invariant differential operators on $G$.
Hausdorff series, Poincaré–Birkhoff–Witt theorem, coexponential map, Duflo isomorphism
An oidification is the universal enveloping algebroid.
N. Bourbaki, Lie groups and Lie algebras
Jacques Dixmier; 1974; Algèbres enveloppantes, Cahiers Scientifique. English translation: 1996; Enveloping Algebras, Graduate Studies in Mathematics 11, American Mathematical Society.
John Milnor, John Moore, On the structure of Hopf algebras, Annals of Math. 81 (1965), 211-264. (pdf)
wikipedia: universal enveloping algebra, PBW theorem
F. A. Berezin, Some remarks about the associated envelope of a Lie algebra, Func. Analysis and Its Appl. 1967, 1:2, 91–102; Rus. original in Fun. Anal. Pril. 1:2 (1967) 1–14 Rus. pdf MR219671
MathOverflow question What is the universal enveloping algebra which is looking for a rather general construction in a class of symmetric monoidal pseudoabelian categories.