Defection, desertion, apostasy mean an abandonment that involves the breaking of a moral or legal bond or tie and that is highly culpable from the point of view of the person, cause, or party abandoned.
Defection emphasizes both the fact of one’s falling away and the loss that is sustained by his failure to adhere to his allegiance; in itself as apart from the context it commonly gives no certain indication of motive, though at times disaffection or loss of confidence is connoted.
Desertion (see also under ABANDON) presupposes an oath of allegiance or a duty or an obligation to guard, protect, or support, the violation of which constitutes a crime or a distinctly blameworthy act. It also suggests a base motive (as cowardly fear or a desire to shirk).
Apostasy implies a repudiation of something one has formerly and voluntarily professed; it connotes therefore a retreat (as in weakness) from a position or stand one has taken.
The term is used chiefly with reference to a repudiation of religious beliefs, but it is employed also when moral, philosophical, or other principles are involved.