In ecommerce context, flash sales are sales which are offered on an ecommerce site at discounted prices for a limited duration of few hours and often for a limited quantity.
As for traditionnal flash sales in retail stores, online flash sales seek to provoke impulse buying and are often used for unsold inventories.
There are two forms of Internet flash sales, flash sales as a business model and flash sales as a promotion and animation tool on traditional ecommerce site.
Daily deals are also a form of flash sale.
Below, an example of flash sale which favors impulse buying by the use of a countdown:
An example of flash sales curator: