Current flowing between two distinct references which are assumed to be at the same voltage, but in fact are not; typically this current flows through unexpected or undesired paths
Ground loops are caused by the same circuit having multiple references, often to ground. If these references are not at the same potential, unanticipated currents will flow between them. These currents can cause noise or damage equipment. As the name implies, this often occurs when "ground" potential is referenced to earth in two different locations, as the earth is not all at the same electrical potential. Similar issues can occur in a PCB, wire harness, or other conductive topology containing loops. Star-grounding may be used to address this issue.