A keyword prefixed with # that categorizes content and (on some platforms) drives discovery via hashtag search and follow.
Hashtags were the primary discovery mechanism on Instagram and Twitter from ~2012 to ~2020. A post tagged with #realestate would appear on the #realestate hashtag page; users following that hashtag would see it in their feed. Hashtags were measurably load-bearing for organic reach.
That era is mostly over. Instagram officially stated in 2023 that hashtags do not boost reach (they only help with categorization). TikTok hashtags work mainly for content categorization to the For You algorithm, not for hashtag-page discovery. LinkedIn hashtags still drive some follow-based reach.
Current practice: use 3–5 highly relevant hashtags (not 30), include at least one branded hashtag (your own), and don't stress about hashtag strategy as a primary lever. The bigger drivers of reach are content quality and the first-30-minute engagement signal.