From Latin: tri- "three" , angulus "corner, angle."

A closed figure consisting of three line segments linked end-to-end.
A 3-sided polygon.

Try this Drag the orange dots on each vertex
to reshape the triangle.

Triangle properties

Vertex

The vertex (plural: vertices)
is a corner of the triangle. Every triangle has three vertices.

Base

The base of a triangle can be any one of the three sides, usually the one drawn at the bottom.
You can pick any side you like to be the base.
Commonly used as a reference side for calculating the
area of the triangle.
In an isosceles triangle, the base is usually taken to be the unequal side.

Altitude

The altitude of a triangle is the perpendicular from the base to the opposite vertex. (The base may need to be extended).
Since there are three possible bases, there are also three possible altitudes. The three altitudes intersect at a
single point, called the orthocenter of the triangle.
See Orthocenter of a Triangle.
In the figure above, you can see one possible base and its corresponding altitude displayed.

Median

The median
of a triangle is a line from a vertex to the midpoint of the opposite side.
The three medians intersect at a single point, called the centroid of the triangle.
See Centroid of a Triangle

It is usual to name each vertex of a triangle with a single capital (upper-case) letter. The sides can be named with a single
small (lower case) letter, and named after the opposite angle.
So in the figure on the right, you can see that side b is opposite vertex B, side c is opposite vertex C and so on.

Alternatively, the side of a triangle can be thought of as a
line segment joining two vertices. So then side b would be called

AC

.
This is the form used on this site because it is consistent across all shapes, not just triangles.

Properties of all triangles

These are some well known properties of all triangles. See the section below for a complete list

There are seven types of triangle, listed below. Note that a given triangle
can be more than one type at the same time.
For example, a scalene triangle (no sides the same length) can have one interior angle 90°, making it also a right triangle.
This would be called a "right scalene triangle".