Emoji reaction
Mark a reaction to a message with an emoji instead of writing a new message. Slack popularized it in chat context (2014); Facebook made it mainstream for posts (2016, replaced plain "Like" with 6 reactions).
Saves thread bloat: 50 replies of "haha" → 50 😂 reactions on the original message. Aggregated count shown. iMessage: tap-back with 6 reactions since iOS 10. Discord: unlimited custom emoji reactions per server. Slack workflows use reactions as lightweight UI buttons (✅ mark done). LinkedIn added "Insightful", "Funny" later; harder adoption than Facebook's. Behavioral research: emoji reactions increase engagement but reduce text replies — quantity vs quality trade-off.