  We define a reflective numerical semigroup of genus $g$ as a numerical
semigroup that has a certain reflective symmetry when viewed within
$\mathbb{Z}$ as an array with $g$ columns. Equivalently, a reflective numerical
semigroup has one gap in each residue class modulo $g$. In this paper, we give
an explicit description for all reflective numerical semigroups. With this, we
can describe the reflective members of well-known families of numerical
semigroups as well as obtain formulas for the number of reflective numerical
semigroups of a given genus or given Frobenius number.
