Ephemeral message
Messages that delete automatically after a timer — 5 seconds, 1 day, 1 week. Snapchat started the category (2011). Signal, WhatsApp, Telegram, iMessage all have disappearing modes today.
Protection: the server doesn't store longer than the timer; the client deletes locally. Not security against screenshots or photo-of-screen — only against accumulated logs on the device. Signal: per-chat timer, can be set from 30 s to 4 weeks. WhatsApp default 24 h. Privacy narrative: "what doesn't exist can't be subpoenaed or breached". Anti-pattern: ephemeral as default creates "lost history" problems for legitimate reference needs.