Synonyms: These nouns denote loss or absence of perfection. A blemish is something thought to mar the appearance or character of a thing: "Industry in art is a necessity—not a virtue—and any evidence of the same, in the production, is a blemish" (James McNeill Whistler).
Imperfection and fault apply more comprehensively to any deficiency or shortcoming: "A true critic ought to dwell rather upon excellencies than imperfections" (Joseph Addison). "Each of us would point out to the other her most serious faults, and thereby help her to remedy them" (Anna Howard Shaw).
Defect denotes a serious functional or structural shortcoming: "Ill breeding . . . is not a single defect, it is the result of many" (Henry Fielding).
Flaw refers to an often small but always fundamental weakness: Experiments revealed a very basic flaw in the theory.