Balance is defined in terms of the impedance of the two signal conductors with respect to a reference, which is usually ground.  If these impedances are equal and non-zero, the system is balanced.  If the impedances are unequal the system is unbalanced. A signal conductor with a grounded return conductor is, therefore, an unbalanced (sometimes referred to as a single ended) system.

Balanced

Balanced connections employ three conductors, one for the positive and one for the negative phase of the signal, and one for the ground. This ensures maximum protection against hum and interference. Servo-balanced connections achieve this electronically, whereas galvanically isolated connections rely on a transformer.

Unbalanced

Unbalanced connections employ two conductors, one for the signal and the other for the ground, and are much more subject to hum and interference than balanced connections. Consumer equipment generally uses unbalanced connections.