One thing I have realized by this point in my life is that words often have semantic meanings.  For example, when I say "I had a great burger this weekend, one person may associate this statement with a hamburger from McDonalds in their mind.  Another person might see one from In-N-Out Burger.  And yet another may see one with bacon and two thick slices of melting cheese oozing out over the bottom bun and still consider that to be a "hamburger."

   
For the purposes of variety and clarity, here are the two best definitions I found of what the Invisible Web is:

            "Plain and simply, the Invisible Web consists of the component parts of the Web that are hard or impossible to find through prominent search engines and directories."

                                               - Karen R. Diaz in "The Invisible Web: Navigating the Web outside Traditional Search Engines"

 Or a longer explanation:

            "In a nutshell, the Invisible Web consists of content that's been excluded from general-purpose search engines and Web directories such as Lycos and Looksmart -- and yes, even Google.  There is nothing inherently 'invisible' about their content.  But since this content is not easily located with the information-seeking tools used by most Web users, it's effectively invisible because it's so difficult to find unless you know where exactly to look."

                          - Chris Sherman and Gary Price, from their article "The Invisible Web: Uncovering Sources Search Engines Can't See"

    Moreover, it should be noted that while most search engines cannot find Invisible Web content, the Invisible Web, in fact, "represents the most structured data on the Web" and is "a point in the spectrum where the content creators invested [the] most effort" (Google Inc.).  It is also estimated that "the [Invisible] Web may be 500 times larger than the fixed Web [content retrievable by search engines]" (Cohen).

    This realization that search engines cannot find every bit of information online may come as a surprise to some Internet users.  It certainly did to me.  In fact, these days many people feel that "internet retrieval is simply [synonymous] with Google" (Egger-Sider and Devine 92).  Hence when trying to find answers for questions as varied as "Where is a good place to get my haircut in Austin, Texas?" to "What field is the idea of transubstantiation rooted in, and what is its history?" one often "googles it."  In fact, using the world "google" when referring to using a search engine to find information has become so ubiquitous in society that on June 15, 2006, "google" officially was added to the Oxford English Dictionary (Wikipedia).

    However, while Google does a good job at finding information, it is not the only way to access information.