Definition from Wiktionary, the free dictionary
- To damage something, especially a surface, in a visible or conspicuous manner.
- To void or devalue; to nullify or degrade the face value.
- He defaced the I.O.U. notes by scrawling "void" over them.
- (heraldry, flags) To alter a coat of arms or a flag by adding an element to it.
- You get the Finnish state flag by defacing the national flag with the state coat of arms placed in the middle of the cross.
- (damage in a conspicuous way): disfigure, mar, obliterate, scar, vandalize
- (degrade the face value): cancel, devalue, nullify, void
Derived terms 
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