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in biology, the failure of an organism to develop properly (see congenital disorder).

in manufacturing, the failure of a product to conform to specification.

in materials science, a crystallographic defect is a structural imperfection in a crystal.

in software engineering, the non-conformance of software to its requirements. A bug, error, are all examples of failures.

to abandon allegiance to one's country and go to another.

in geometry, the amount by which the sum of the angles at a vertex of a polyhedron falls short of a complete circle. If the sum of the angles exceeds a full circle, then the defect is negative. See defect (geometry).

a crystallographic defect is a site where a crystal lattice is imperfectly formed.

in politics and/or a military context, the verb to defect refers to switching sides (see defector).