Push refers to a system in which data is "pushed" to a device by a server rather than "pulled" or "fetched" after a request by the user. The server initiates the data transfer when new content is available without any action taken by the client device.
One common use for push technology (also called "server push") is to automatically deliver email to a user's device as the server receives it instead of waiting for the user to check for new messages. Messaging services like SMS, iMessage, and WhatsApp also push data to a user's device instantly without waiting for a request.
Apple, Microsoft, and Google include support in their mobile and desktop operating systems for push notifications. These notifications allow a remote server to push information through an application you have installed into the operating system's notification center. Web browsers like Chrome, Firefox, Edge, and Safari also support the web push protocol, which lets websites send push notifications through the browser to the user's notification center. Thankfully, websites cannot send notifications without your permission first.
NOTE: Mobile operating systems like iOS and Android also support special civic push notifications, which can send severe weather warnings, missing person alerts, and other emergency notifications to people in a local area.