Why is the ninth month called September?
early 15c., from Middle French defect and directly from Latin defectus "failure, revolt, falling away," noun use of past participle of deficere "to fail, desert" (see deficient).
1570s, from Latin defectus, past participle of deficere "to fail, desert" (see defect (n.)). Related: Defected; defecting.
defect de·fect (dē'fěkt', dĭ-fěkt')
A lack of or abnormality in something necessary for normal functioning; a deficiency or imperfection.
imperfection in the regular geometrical arrangement of the atoms in a crystalline solid. These imperfections result from deformation of the solid, rapid cooling from high temperature, or high-energy radiation (X-rays or neutrons) striking the solid. Located at single points, along lines, or on whole surfaces in the solid, these defects influence its mechanical, electrical, and optical behaviour.