The term Cyber Monday refers to the Monday immediately following Black Friday; the first Monday after Thanksgiving Day. It is considered the ceremonial kick-off of the online holiday shopping season. Whereas Black Friday is associated with traditional brick-and-mortar stores, "Cyber Monday" symbolizes one of the busiest days of the year for online retailers although the busiest online shopping day of 2005 was actually December 12.

The idea was that consumers would return to their offices after the Black Friday weekend and then make online purchases – buying all of the stuff they didn't find in their local stores.

Cyber Monday has evolved into a significant marketing event, sponsored by the National Retail Federation's Shop.org division, in which online retailers offer low prices and special "today only" promotions. Here are a few "current" examples:

Overstock.com

The term "Cyber Monday" was not used much within the ecommerce community until the 2005 holiday season. According to Scott Silverman, the Executive Director of Shop.org, the term was coined based on research revealing that online retailers reported a significant increase in sales the Monday after Thanksgiving in 2004. Ecommerce sites report that the busiest shopping days usually fall between December 5-15 in any given year. 

Amazon is leading the pack and their Kindle promo is flying off of the shelves:

UPDATE: NOV. 30, 2007

Online shoppers in the U.S. set a record for one-day sales on Cyber Monday, racking up US$733 million in purchases, comScore Networks said Tuesday.
 
The figure represents a 21 percent increase over the same day last year and the most money that U.S. shoppers have ever spent in a single day online, according to comScore.