An unbalanced three phase system is opposite to a balanced three phase system. Its voltages and currents doesn't have the same amplitude and aren't phase shifted by 120°.
A balanced three phase system is an electrical power system which uses three wires for generation, transmission and distribution. It can be wye connected or delta connected. Each phase has 120 degrees apart each other, in other words, balanced in current and powers. It also has balanced impedance load and a balanced voltage source.