''Tri'' means Three, so Triangle is a polygon which have 3 angles, 3 side and 3 Vertices.
In order words:
Triangle is a simple closed curved made up of three line segments.
Following are some more topics which you can study:
Properties of Triangle
Property 1 = Sum of Two Sides
Property 2 = Angle Sum Property of Triangle
Types / Classification of Triangle
Right Angled Triangle
Acute Angled Triangle
Obtuse Angled Triangle
Equilateral Triangle
Isosceles Triangle
Scalene Triangle
Perimeter of an Equilateral Triangle
Median of a Triangle
Interior of a Triangle
Exterior of a Triangle
Altitude of a Triangle
Interior Angles of a Triangle
Exterior Angles of a Triangle
Interior Opposite Angles of a Triangle
(Study these topic in detail with examples from the above respective links)

