noun, の adjective
When you have something with three angles, what shape is that? It's a triangle.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Please go buy triangle crackers.
I am stuck in a love triangle now.
Canadians must have been inspired by Picasso quite a bit. Square boxes, round pizzas, and triangle pizza slices are everywhere.