Disappearing message
Messages that delete themselves automatically after a set time or after being read. Snapchat built its entire identity on the idea; today it's in Signal, WhatsApp, Instagram, and Messenger as a privacy and "ephemeral communication" feature.
Mechanic: the sender (or the chat) sets a timer — seconds, hours, days — and the message disappears from both devices when the time is up. Win: less digital footprint, more relaxed communication ("this doesn't need to be saved"), and a privacy protection. Limitation: the recipient can often take a screenshot (some apps then warn), so it doesn't protect against a determined counterpart. Snapchat (2011) made ephemeral communication the norm for a whole generation as a reaction against the internet's "everything is saved forever". Signal uses it for serious privacy. Related to view once and vanish mode.