Gee, I am surprised I didn't get an email on this one.
The formal technical term in the US would be a "knee brace".
Buttress is not accurate and refers to something different.
I have found that some structural items in different countries have different names, so locally it may be called some other kind of brace, but it is still a brace.
A buttress, or a flying buttress, is a stiffening member that transfers horizontal forces into the foundation.
like
Yeah, saw that when I looked it up on Google Images ... suddenly staring at cathedrals ...
Glad I ain't in construction, buttres, kneebrace, gallow brace, gusset beam, driving me up the wall!

I'm beginning to see how it is referred to as "The thingy that holds all up" LMAO