An Allow List is a set of email or IP addresses that have been granted access to to a service, or a list of web sites that a user is permitted to access.
Allow listing — some services use the term “whitelisting” but this is rightly falling out of usage — refers to the process of granting a specific IP address access to a server or system. SendGrid’s IP Access Management security feature makes use of Allow Lists, for example: it lets you specify who can access your SendGrid account by adding their IP address to an Allow List.
You may also encounter the term Allow List when reading a list of IP addresses that receive special treatment when sending mail to a specific inbox provider. However, email Allow Lists are becoming less and less common. These days, the best way to ensure your messages land in the inbox is to send mail that you know your recipients will love.