Defect can refer to:

• 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, defect, 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).