Lattices in the sense of discrete subgroups of Euclidean spaces, as used in number theory, discrete geometry, Lie groups, etc. (Not to be confused with lattice theory or lattices as used in physics!)
This tag is ambiguous, so we recommend against its use. See http://meta.mathoverflow.net/a/1761/121
Questions with this tag tend to be about either partially ordered sets, or discrete subgroups of Lie groups, or finite rank free abelian groups equipped with a quadratic form.