> This entry is about the notion of lattice in group theory/quadratic form-theory. For other notions see at lattice (disambiguation).
Classically, a lattice in the Cartesian space $\mathbb{R}^n$ is a discrete subgroup (of the underlying topological abelian group) that spans $\mathbb{R}^n$ as a vector space over $\mathbb{R}$. This may be generalized, from $\mathbb{R}^n$ to a general locally compact abelian group.
A lattice in a locally compact Hausdorff abelian group $A$ is a subgroup $L \hookrightarrow A$ that is discrete and cocompact, meaning that the quotient group $A/L$ with the quotient topology is compact.
Applying Pontryagin duality, the dual of the quotient map $q: A \to A/L$ is in that case a discrete subgroup $\widehat{A/L} \hookrightarrow \widehat{A}$ that is also cocompact (its cokernel being the compact group $\widehat{L}$). This is called the dual lattice of $L$.
Notable examples of classical lattices (in $\mathbb{R}^n$) include
The standard diagonal inclusion of a global field (such as a number field) $k$ into its ring of adeles $A_k$ is a lattice in the more general sense. Recalling that $A_k$ is Pontryagin dual to itself in a standard way, the lattice $k$ is identified with its dual lattice.
A. E. Brouwer, Lattices, Course notes (2002) (pdf)
John Conway, N. Sloane, Low dimensional lattices I: Quadratic forms of small determinant
Wikipedia, Lattice (group)
Wikipedia, Lattice (module)