A great way to boost your search engine ranking is to include relevant keywords and text on your website. But go about this the wrong way and you could find your entire website removed from the search engine index. Hidden text is one of the oldest spamming techniques used by search engine optimizers who try to artificially increase the keyword density of their web pages.
Keyword density refers to how many times a specific keyword is included on any web page. In order to maintain a good search engine ranking, website owners should include these keywords throughout the page while maintaining readability for the human visitor.
The ‘black hat’ tactic of hidden text attempts to increase keyword density by placing a large block of unreadable text on a web page. While unreadable to the human visitor, the resulting text is still scanned and picked-up by search engine spiders. A common technique for hiding text is to include it in the same or a similar colour to the background of a page. The Wikipedia entry for ‘hidden text’ has an example of the practice which you can check out here.
Search engine spiders have long been aware of this technique and ensure web pages using this method are dropped from the search index. They may also penalize the entire site even if only one page contains hidden text – so watch out! The overwhelming message here is don’t use hidden text of any kind on your web pages; it will do you more harm than good. On a side note, if you’re keen to see a funny example of this technique, visit the blog of Google’s Matt Cutts.