WebSocket
A protocol for two-way real-time communication between browser and server.
Different from HTTP: HTTP is request → response → done. WebSocket opens a persistent connection where both sides can send messages whenever. Good for chat, live updates, multiplayer games.
Starts as an HTTP handshake, then "upgrades" to the WebSocket protocol. Uses port 80 (ws://) or 443 (wss://) — passes through most firewalls.