This document defines key properties of triangles including: vertices, bases, altitudes, medians, angles, and different types of triangles such as isosceles, equilateral, scalene, right, obtuse, acute, and equiangular triangles. It also defines Heron's formula for calculating the area of a triangle when the lengths of all three sides are known. The document was created by Shreyansh Jain and Varun Israni to explain triangle properties.