Noun
English Wikipedia has an article on:keepaliveWikipedia
keepalive (plural keepalives)
(electronics, communication) A short message sent by one device to another to check that the link between the two is operating, or to prevent this link from being broken due to inactivity.
In the "Established" state, the router can send/receive: Keepalive; Update; and Notification messages to/from its peer. Source: Internet
Keepalive messages are exchanged and, upon successful receipt, the router is placed into the "Established" state. Source: Internet
The NGINX directives were used to quickly close keepalive connections. Source: Internet
Your points about DDoS and performance are correct, but there is a fairly simple way to use a keepalive timeout for performance without worrying that attackers will open more TCP connections than your server can handle. Source: Internet