Cyber Monday is a marketing term for e-commerce transactions on the Monday after Thanksgiving in the United States. It was created by retailers to encourage people to shop online. The term was coined by Ellen Davis of the National Retail Federation and Scott Silverman, and made its debut on November 28, 2005, in a Shop.org press release entitled "Cyber Monday Quickly Becoming One of the Biggest Online Shopping Days of the Year." Cyber Monday takes place the Monday after Thanksgiving; the date falls between November 26 and December 2, depending on the year.

Cyber Monday has become the online equivalent to Black Friday and offers a way for smaller retail websites to compete with larger chains. Since its inception, it has become an international marketing term used by online retailers around the world.

The shopping event, which originated in 1950 with retail stores lowering prices and extending hours, evolved into a global online phenomenon in 2000, boasting one of the highest worldwide sales, and subsequently inspired the creation of Cyber Monday in 2005. According to the Shop.org/Bizrate Research 2005 eHoliday Mood Study, "77 percent of online retailers said that their sales increased substantially on the Monday after Thanksgiving, a trend that is driving serious online discounts and promotions on Cyber Monday this year (2005)".

In 2017, Cyber Monday online sales grew to a record of $6.59 billion, compared with $2.98 billion in 2015, and $2.65 billion in 2014. However, the average order value was $128, down slightly from 2014's $160. The Cyber Monday on November 30, 2020, (the first during the COVID-19 pandemic) was the biggest online shopping day in US history with a total of $10.7 billion in online spending.

The term "Cyber Monday" was coined by Ellen Davis, and was first used within the ecommerce community during the 2005 holiday season. According to Scott Silverman, the head of Shop.org, the term was coined based on 2004 research showing "one of the biggest online shopping days of the year" was the Monday after Thanksgiving (12th-biggest day historically). Retailers also noted the most significant shopping period was December 5 through 15 of the previous year. In late November 2005, The New York Times reported: "The name Cyber Monday grew out of the observation that millions of otherwise productive working Americans, fresh off a Thanksgiving weekend of window shopping, were returning to high-speed Internet connections at work Monday and buying what they liked." At the time, a lot of people had slow Internet at home. The idea for having such a holiday was created by Tony Valado, in 2003 while working at 1800Flowers.com, and coined "White Wednesday" to be the day before Thanksgiving for online retailers.

Beginning at 7 pm AEDT on November 20, 2012, Australian online retailers decided to hold a similar event for the first time, dubbed "Click Frenzy". Many websites immediately crashed, went offline, or had major server issues, including the Click Frenzy promotion website. David Jones, a major retailer, ran a competing sale dubbed 'Christmas Frenzy' on the same date.

In Belgium, Cyber Monday has gained popularity since 2016. Due to efforts off several major online shops launching large Cyber Monday campaigns, the average revenue during Cyber Monday has increased with 50% compared to the 2015 edition.

Cyber Monday came to Canada in 2008. The National Post featured an article published on November 25, 2010, stating that the parity of the Canadian dollar with the US dollar caused many Canadian retailers to have Black Friday and Cyber Monday sales of their own. According to the article, an estimated 80% of Canadians were expected to participate in Black Friday and Cyber Monday sales. Speculation has been made that with all major US television broadcasters—which are typically available to Canadians—emphasizing Black Friday and Cyber Monday sales for stores that are also doing business in Canada, Canadian retailers needed to mimic sales offerings in order to keep Canadian dollars from being spent in the US.