I've been unable to find this specific piece's name, just for the reason that it is a very unorthodox piece.
Informally, it's known as the Nexogon.
Bricklink calls it "Plate, Modified 6 x 6 Hexagonal with Pin Hole". The part number is 27255. It's original use was as a shield in the Nexo Knights line in 2017, but it has appeared in a few other sets since then.
If you're interested in ways to use this part, New Elementary did a whole series of posts from different builders when it was first released. The unique shape and different connection families open up a lot of fun opportunities.
This is what I managed to create with the Nexogons: