Cover or Legend:
A spy's false identify can take years to build and seconds to destroy. The importance, danger and duration of the mission determine how much care is invested in the creation of an identity.
A "cover" is usually part of a quick disguise. It can be as simple as a false name. A "legend" is a carefully developed artificial life history and background requiring painstaking attention to detail. A spy may live a false identify for years, establishing the legend in preparation for an operation.