Subgroup of index two

Symbol-free definition
A subgroup of a group is said to be of index two if its index in the group is two, or equivalently, if it has exactly one coset other than itself.

Definition with symbols
A subgroup $$N$$ of a group $$G$$ is said to be of index two if $$[G:N] = 2$$.

Formalisms
The property of being a subgroup of index two can be expressed in first-order logic (in fact, the property of being a subgroup of any fixed finite index can be expressed in first-order logic).

Weaker properties

 * Normal subgroup:
 * Subgroup of prime index
 * Abelian-quotient subgroup