A hash key, sometimes referred to as a pound key in the United States. Most commonly used online to make a hashtag (or hash flag) on social media sites.
The Keycap Number Sign emoji is a sequence of the # Number Sign and ⃣ Combining Enclosing Keycap emojis.
These display as a single emoji on supported platforms.
Keycap Number Sign was approved as part of Unicode 3.0 in 1999
and added to Emoji 3.0 in 2016.