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.
4So beautiful, that is Dec 2, 2017 at 0:32
1Is there any name for that type of irregular hexagon? I couldn't find any in MathWorld. Also, here's more fan stuff on Nexogon:brothers-brick.com/2017/01/19/… Dec 2, 2017 at 4:59
2@smci: You can see this hexagon as a truncated equilateral triangle. Dec 2, 2017 at 10:28
Thank you so much! I have been having such a hard time trying to identify this piece, but you made my job so much easier. Thanks again! Dec 4, 2017 at 14:44
This is what I managed to create with the Nexogons:
Flickr Album: https://www.flickr.com/photos/162108633@N02/albums/72157688876724660