A turncoat is a person who shifts allegiance from one loyalty or ideal to another, betraying or deserting an original cause by switching to the opposing side or party. In political and social history, this is distinct from being a traitor, as the switch mostly takes place under the following circumstances:

• In groups, often driven by one or more leaders.
• When the goal that formerly motivated and benefited the person becomes (or is perceived as having become) either no longer feasible or too costly even if success is achieved.

From a military perspective, opposing armies generally wear uniforms of contrasting colors to prevent incidents of friendly fire. Thus the term "turn-coat" indicates that an individual has changed sides and his uniform coat to one matching the color of his former enemy. For example, in the English Civil War during the 17th century, Oliver Cromwell's soldiers turned their coats inside out to match the colors of the Royal army (see Examples below).