What is "Deface/Defacing"?

According to dictionary the word "defacing" means to wreck something.Likewise this,in the term of hacking the word means to change any content to hackers wanted contents.Most often a hacker changes the home(index.html/index.php) files contents to his message to the admin.often he is seen replacing it with image or text file.This is called defacing.the page(html/php) that is replaced by the hacker is called "Deface Page"