Malicious website modification by a hacker.

The criminal act of altering the contents of a web site without permission and conducting the alterations in a manner that vandalizes the site or embarrasses the sites owner.

Modifying the content of a website so that it is "vandalized." Normally this is done to be embarrassing to the owner.

That which mars or disfigures.

the act of damaging the appearance or surface of something; "the defacement of an Italian mosaic during the Turkish invasion"; "he objected to the dam's massive disfigurement of the landscape"

The act of defacing, or the condition of being defaced; injury to the surface or exterior; obliteration.

In common usage, to deface something refers to marking or removing the part of an object (especially images, be they on the page, in illustrative art or as a sculpture) designed to hold the viewers attention. Example acts of defacement could include scoring a book cover with a blade, splashing paint over a painting in a gallery, smashing the nose of a sculpted bust. Iconoclasm led to the defacement of many religious artworks.

Defacement is a term used in heraldry and vexillology to refer to the addition of a symbol to another flag. For example, the Canadian Red Ensign was the British Red Ensign defaced with the shield of the Arms of Canada.

Changing the home page or other key pages of a Web site by an unauthorized individual or process.

(Context: computer security) Any unauthorized change to a web page.

To destroy the external appearances