Also known as an ICO Whitelist, a Whitelist is technically a list of interested participants or investors in an ICO. These individuals are registered with a clear intent of taking part in buying the coin.
ICO Whitelists were actually born out of the vexation with huge ICOs and Initial Exchange Offerings (IEOs) that are disclosed in the worldwide web, despite the token sales already ending. As this is the case, these Whitelists guarantee that access will be given to the ICO sale.
Whitelistings are commonplace in the market, especially upon the opening of a sale. Whitelists may be strict or lenient depending on what is being sold.